Whether they got the prisoner to talk is obvious.
Is he talking? Then you got him to talk. (Mechanically you can handle this several ways, but the simplest is to just assume that they crank up the pressure until he talks. Unless you're under very tight time limits or the prisoner is likely to escape, there's not really any other outcome.)
Whether he's lying, or holding something back, is not obvious.
This is where your players learn an important fact about torture.
Before we roll skill checks we should know what success and failure look like. The interrogation is going to continue until the party believes they've gotten the answers they want from the prisoner. Success means that those answers are correct. Failure means they're incorrect or incomplete.
If the guy has no reason to lie or hold back, then they can't fail. Just tell them everything he knows.
Suppose he does want to lie. That's a Deception check. Since the PHB doesn't spell this out: the DC for a Deception check to hide information is the Passive Insight of whoever you're talking to. Hopefully the players will be smart enough to have their highest-Insight guy involved in the interrogation.
(If anyone asks if they can "make an Insight check", I recommend giving a steely glare and asking what exactly their character is doing to "gain insight".)
The Deception check should be a hidden roll. If it fails, then you tell the player a plausible lie. If it succeeds, tell them the truth. Either way, tell them they're pretty sure from the guy's body language and tone of voice that it's true. This summarizes the whole process of asking questions, getting lied to or distracted, scaring the guy again, asking more questions, etc. into one roll that determines the outcome we care about.