Veritaserum

Veritaserum was a powerful truth serum. The potion effectively forced the drinker to answer any questions put to them truthfully, though there were certain methods of resistance.

Nature
Veritaserum was clear, colourless, and odourless and was almost indistinguishable from water. According to Severus Snape, the potion had to mature for a full lunar phase, and was quite difficult to produce; it was something Horace Slughorn expected his students to be able to produce once they finished their N.E.W.T.-level Potions. It could be mixed with pretty much any drink, and three drops was a sufficient dose to make the drinker "spill out his innermost secrets". Thus, the potion forced the drinker to tell the complete truth to any question put to them, according to what the drinker perceived as true.

Limitations
Since some wizards and witches could resist its effects while others could not, Veritaserum was "unfair and unreliable to use at a trial", and would have been difficult to use as definitive proof of guilt or innocence.

Another problem was that the victim stated what only they believed to be true, so the victim's sanity and perception of reality were also factors in during interrogations. Therefore, while the drinker's answers were sincere, they were not necessarily true. This was the main reason why Barty Crouch Jnr's testimony was only partially credible, as some of his answers were true in his mind, but known to be false by his interrogators; Cornelius Fudge believed that Crouch being a "raving lunatic" was a mitigating factor on the Veritaserum's full effectiveness, thus chose not to believe half of it. However, despite Crouch's sociopathic and unhinged tendencies, his grip on reality and testimony was just.

Using Veritaserum on a student was strictly forbidden, at least in Hogwarts, a prohibition that Severus Snape considered "regrettable."

Resistance
Despite being the most powerful truth serum in existence, it could still be resisted through different methods, including the taking of its antidote and Occlumency.