How do you know who you are really talking to?

Question

Throughout the series we have seen many scenarios in which someone acts as someone else. Barty Crouch Jr. transforms into Mad-Eye Moody, Harry and Ron transform into Crabbe and Goyle, and Hermione transforms into Bellatrix. This is a huge flaw in the series, as you potentially have no idea who you are talking to, or oblivious to the fact that someone could be using Polyjuice Potion, to transform into another.

My question is, how do you know who you are really talking to? It seems silly (before Voldemort took over the Ministry) that you must ask everyone a secret question to identify them. We see Remus do this to Harry in the seventh book/movie.

How do we know that Dumbledore was really Dumbledore, or Severus was really Severus? And the same for everyone else?

Answer 1

Polyjuice potion can and has been used for espionage, as you've stated, but its use is highly situational.

Even assuming any competent witch or wizard can brew it with the right tools, there are still a few reasons that it's impractical for casual use:

• It takes one month to brew, which means its use must be planned far in advance.
• It requires pieces of the person to be transformed into. You're probably not going to have someone's hair or skin unless you've already had contact with them before.
• It only lasts for a few hours. For extended use, you will need a regular brewing schedule and to somehow have an unlimited supply of hair or other body pieces. It's hard to do that unless the person in question is dead or held securely. Barty Crouch Jr. managed this, but it is not typical.
• There's a risk of the real person blowing your cover if you didn't incapacitate them.
• Possibly the most important point is that you need to be convincing as the transformed person, or the potion is useless. There are very few people who could manage to fool the target's family and friends for any extended period of time.

I believe that while maybe anybody could make the potion, very few could use it effectively or would even have reason to when easier options are available. If it's not used often, there's usually no reason to believe you're talking to an impostor. If you do have reason to believe, you can ask questions as Lupin did.

Answer 2

You ask questions to the people you are interacting with that only they would know. This happens a few times in the later books. Here is a quote after the battle of the seven Potters.

No sooner had Mrs. Weasley bent over her son that Lupin grabbed Harry by the upper arm and dragged him, none too gently, back into the kitchen, where Hagrid was still attempting to ease his bulk through the back door.

"Oi!" said Hagrid indignantly, "Le' go of him! Le' go of Harry!" Lupin ignored him.

"What creature sat in the corner the first time that Harry Potter visited my office at Hogwarts?" he said, giving Harry a small shake. "Answer me!"

"A – a grindylow in a tank, wasn't it?"

Lupin released Harry and fell back against a kitchen cupboard. "Wha' was tha' about?" roared Hagrid.

"I'm sorry, Harry, but I had to check," said Lupin tersely. "We've been betrayed. Voldemort knew that you were being moved tonight and the only people who could have told him were directly involved in the plan. You might have been an impostor."

"So why aren' you checkin' me?" panted Hagrid, still struggling with the door.

"You're half-giant," said Lupin, looking up at Hagrid. "The Polyjuice Potion is designed for human use only."

Answer 3

There is one magic item that is supposedly impossible to fool. At least by the means introduced in canon:

The Marauder's Map

This map, when activated, displays the real name of anyone walking around Hogwarts. We know from the Prisoner of Azkaban and Goblet of Fire respectively that the map will not be fooled by an Animagus or Polyjuice Potion.

Also this map was made by students. Even if we assume that James, Sirius and Remus were very gifted students, it's not very likely that they are the only persons in the wizarding world capable of this.