Can a wizard or witch recognize a Maledictus? What about a Parselmouth? Can they, without asking the serpent, recognize if it once had human form?
No, they can't, or at least not just by looking.
In Crimes of Grindlewald it is revealed that Nagini was a Maledictus, and in the original movie series several wizards - including Harry, who was a Parselmouth - encountered Nagini without ever realizing she was anything other than a snake.
There are probably spells that could reveal the truth, if the wizard became suspicious. But it isn't something a wizard can just sense, in the way a powerful wizard like Dumbledore can just sense the presence of magic in an apparently natural setting.
... that does still leave the possibility that an experienced Parselmouth, one who knows about and has encountered several snake Maledictuses previously, might be able to spot some tell-tale signs. But as Maledictuses are very rare, and they don't all take the same form anyway, this seems unlikely to happen.
Probably not - they can’t with Animagi.
There isn’t enough information on Maledictuses to state conclusively whether wizards could tell one from a regular animal on sight. No one in the original Harry Potter books seemed to ever suspect Nagini was anything but a snake, but her case may not be a typical example of a Maledictus, as she was also owned by the Dark Lord and turned into one of his Horcruxes, so he would have magically altered her by then. It isn’t shown yet whether wizards recognize her as a Maledictus before she came into the Dark Lord’s possession. However, what is known is that wizards don’t recognize Animagi (who, though different from Maledictuses, are also humans transformed into animals) on sight. Because it can easily be used as a disguise, Animagi must register their animal form with the Improper Use of Magic Office.
“It takes years to become an Animagus, and then you have to register yourself and everything,’ said Hermione vaguely, now squinting down the index of Weird Wizarding Dilemmas and Their Solutions. ‘Professor McGonagall told us, remember … you’ve got to register yourself with the Improper Use of Magic Office … what animal you become, and your markings, so you can’t abuse it …”
- Harry Potter and the Goblet of Fire, Chapter 26 (The Second Task)
Wizards can recognize Animagi if they know their animal form, but if they don’t, it’s very unlikely they’ll recognize them on sight as Animagi rather than ‘real’ animals.
“Harry, however, continued to stare at Sirius. ‘What if they catch you? What if you’re seen?’
‘You three and Dumbledore are the only ones round here who know I’m an Animagus,’ said Sirius, shrugging, and continuing to devour the chicken leg.”
- Harry Potter and the Goblet of Fire, Chapter 27 (Padfoot Returns)
Therefore, without any specific information on whether wizards can recognize Maledictuses on sight, for now the most logical speculation would be that they cannot, similarly to how they’re unable to tell Animagi from regular animals just by looking at them.