In Harry Potter, was there a spell or magical tool to detect that a wizard was pure-blood, half-blood or muggle-blood? Or, did they simply use family information of a wizard? Was it possible for a wizard to hide his/her blood-status?
I think JKR contradicted herself here.
On one hand, there are hints that blood status is detectable, as @Slytherincess and @Kristen's answers state.
On the second hand, if so, there would be no point in Umbridge-chaired hearings to determine blood status that Hermione witnessed in DH when they infiltrated the Minitry - the hearing allowed people to "prove" their blood status by providing documentation.
If you can determine someone's blood status magically, documentation and hearings are 100% superfluous and un-needed.
"No," said Umbridge, "no, I don't think so, Mrs. Cattermole. Wands only choose witches or wizards. You are not a witch. I have your responses to the questionnaire that was sent to you here – Mafalda, pass them to me."
... A pity," she continued in a louder voice, flicking through Mrs. Cattermole's questionnaire, "that the same cannot be said for you. ' Parents professions: greengrocers'."
(Yes, I know the out-of-universe reason to show the hearing... my point is that it's contradictory from in-universe perspective).
It's possible that if there is a way of genetically or magically testing for pure blood lineage and Umbridge et al don't use it because many of them are lying about their ancestry. Hermione says that the Death Eaters can't all be pure bloods because there aren't enough pure bloods left, so most of them are just half-bloods pretending to be pure-bloods, so I can't imagine that would be particularly popular among death eaters, and although we don't know about Umbridge's blood status specifically, if she's lying about being related to the Selwyns, she must be a little insecure about it. And of course, Voldemort wouldn't want people prying too closely into his ancestry.
It also seems like the trials are being used by some ministry employees, at least, for political gain. Someone congratulates Harry-Runcorn on one of his victims because the interlocutor is "almost confident [he'll] get his job now." That makes it seem like the convenience of the sham trials is really important to the way the ministry functions-- you can disenfranchise people you don't like and let your supporters slide.
That's not to say that I don't think there's a lot more evidence on the you-can't -really-tell side. There's no concrete evidence for being able to tell, just hints and allusions and possibilities. The trials seem like good evidence that you can't tell. But it just seems odd that there isn't a spell, since magic and genetics play so nicely together elsewhere (Polyjuice potion needs a bit of the person, which seems like DNA, Hermione's antidote in Slughorn's class includes a bit of her own hair, jinxes don't work on people with giant-DNA, Harry's patronus matches James's animagus form, JK Rowling has stated that all muggle-borns have a witch or wizard ancestor sometime in the past), so I wanted to offer this one other possible explanation, just as food for thought.
I think it's possible to detect blood status without knowing family history, based on something Draco Malfoy says in Goblet of Fire:
‘Language, Weasley,’ said Malfoy his pale eyes glittering. ‘Hadn’t you better be hurrying along, now? You wouldn’t like her spotted, would you?’
He nodded at Hermione, and at the same moment, a blast like a bomb sounded from the campsite, and a flash of green light momentarily lit the trees around them.
‘What’s that supposed to mean?’ said Hermione defiantly.
‘Granger, they’re after Muggles,’ said Malfoy. ‘D’you want to be showing off your knickers in mid-air? Because if you do, hang around ... they’re moving this way, and it would give us all a laugh.’
‘Hermione’s a witch,’ Harry snarled.
‘Have it your own way, Potter,’ said Malfoy, grinning maliciously. ‘If you think they can’t spot a Mudblood, stay where you are.’
Goblet of Fire - page 110 - Bloomsbury - chapter 9, The Dark Mark
Draco doesn't specify exactly how the Death Eaters might have been able to tell that Hermione is Muggleborn without knowing her heritage, but he does definitely suggest it's possible.
As for hiding one's blood status, the fact that Bellatrix Lestrange, in Order of the Phoenix, didn't know that Voldemort was a half-blood seems to indicate it's certainly possible to fool others. Bellatrix was rabidly elitist; when Harry informed her that Voldemmort was a half-blood she was not only disbelieving, but also fearful at the mere suggestion. I won't type in another wall of text to provide the passages, but it's in chapter 36, The Only One He Ever Feared, in Order of the Phoenix.
I would think that perhaps there was a way of magically determining the blood status. If they can detect muggleborn children for Hogwarts, I'm sure they have a list somewhere of each status, at least until they are 17. Those that may be 'half blood' or have a muddy history, might need papers to prove their background.
It was never really touched on.
It is difficult to say but what we know for sure (from book 7 mainly) is that :
Muggle born registration was mandatory during the second raise of Voldemort and that people who wanted to avoid it had to go on the run
Muggle born weren't allowed to go into Hogwarts during the second raise of Voldemort (all students were required to prove their blood status)
When a muggle got his/her letter from Hogwarts, a specialized wizard/witch was sent to explain the situation to the parents
Those information could led to two different deductions.
And we do not know about foreign wizards/witches : could they lie or not ? Produce fake papers ? It is not explained in the books.
Rowling has said that magic is inherited, and another user here has theorized one of the ways to get the complex mixture of muggle-borns, squibs, and skipping several generations.
This makes it possible for both to be true, depending on how the magic works:
Let's say muggle-borns/muggle/pure-blood detection is done by detecting some combination of "magic-positive" genes. The specific combination they detect may not correspond perfectly to actual ability, but simply have a very high success rate.
This would mean that they can separate pure-bloods from others, as Malfoy says, but perhaps only with a 99% success rate. Because of that last 1%, the hearings would then be necessary to weed out anyone who either was pure-blood (and the test has false positives) or wasn't pure-blood (and the test has false negatives).
This extra catch, to keep pure-bloods from being removed from the general populace, would be considered necessary due to the low number of pure-blood families still in existence.
If I recall correctly, Umbridge was using the Horcrux locket as her proof that she came from a pure-blood family - which was obviously false, since the locket wasn't hers. This means that it would indeed be possible to cover up the truth, by taking advantage of that 1% failure rate.
No, it doesn’t seem to have been possible to detect blood status.
When the Dark Lord is trying to weed out the Muggle-borns, instead of using any magical way of detecting the possible Muggle-borns’ blood status, they make them show up for questioning.
“Nevertheless, unless you can prove that you have at least one close wizarding relative, you are now deemed to have obtained your magical power illegally and must suffer the punishment.”
- Harry Potter and the Deathly Hallows, Chapter 11 (The Bribe)
They make accused Mudbloods answer questions about themselves, which the accused would have reason to lie on, which would be a much less reliable way of testing than using a detection spell. As for those working for the Dark Lord who were obscuring their own blood status, it’s unlikely they’d be subject to the test themselves so there wouldn’t be much reason not to use it.
But, it was possible there could be a blood-status related spell.
However, there is an indication that there could be some enchantment that knows blood status. Dumbledore thought it was possible that there was some sort of enchantment on the Black family home, Number 12 Grimmauld Place, to prevent it being owned by someone who’s not pure-blood.
“While his will makes it perfectly plain that he wants you to have the house, it is nevertheless possible that some spell or enchantment has been set upon the place to ensure that it cannot be owned by anyone other than a pure-blood.”
- Harry Potter and the Half-Blood Prince, Chapter 3 (Will and Won’t)
There wasn’t any such enchantment on Grimmauld Place, though, as it and Kreacher are inherited by Harry. That Dumbledore thinks there might be, it implies it’s at least possible to do such a thing. However, it’s not said how this enchantment was supposed to have worked. It’s possible it wouldn’t have actually detected blood status, but somehow detected ‘allowable’ people instead, for example only people who are on the Black family tree and haven’t been blasted off.
I imagine, but have neither citation or other proof, that any means of "detecting" non-pure-bloods, is as reliable as gaydar.
I also imagine such a "wiz-dar" detector going by Snape and calling him a Blackadder...and getting complimented on his astute perception, then threatened severely should such perception be shared with anyone else.
I think it’s still an unclear picture, however there has to be some way of identifying muggleborns. I simply believe that they play it by ear or common knowledge.
The sacred 28 consists of 28 of the purest and ‘untainted’ pureblood families, each with its own line of witches and wizards. However, not all pureblood families are recognised upon this list. The Potters for instance, are not mentioned despite being a long line of pure blooded families. It is because other pureblood families believed the Potters to be decendants from Muggles, as potter is a common muggle last name. So from this we can see that wizards rely a lot on last names to decipher or guess blood purity.
As for identifying Muggle borns, I would assume it to be of a similar basis. Names such as ‘Granger’ and ‘Evans’ would have potentially been last names half bloods and pure bloods had never heard before, so would the Wizards and witches henceforth assume that they were muggle born? What if say for instance, an witch had married into a muggle family and taken the last name? would the child they may have be suspected muggleborn until told otherwise most likely through other people.