Torture is by definition neither open (you're not going to tell the public "Oh we just tortured X") nor idependently verifiable (for this multiple entities would need to torture the same person for the same information), so this kind of evidence would be really hard to obtain.
Also given the fact that you can't know whether the person you're torturing for intelligence actually knows anything, they might (and probably will) eventually tell you "something" you want to hear, just in order for the torture to stop. At this point though, you'll don't know whether the information is actually viable, you still have to confirm it somehow, which might be difficult depending on what kind of information it is. In case of "The weapons are hidden at X" this is relatively easy to do, you send some agents to check that place out. In the case of "Who do we need to drone strike?" it's not as easy, because you can either kill them or not.
Back in Roman and Medieval times, torture was not considered inappropriate. There was no need to conceal it. Typically the accused was assumed to be guilty, and torture was a just a way to get him to confess to what the court already "knew" was true. Otherwise the trial could not proceed.
Also given the fact that you can't know whether the person you're torturing for intelligence actually knows anything, they might (and probably will) eventually tell you "something" you want to hear, just in order for the torture to stop. At this point though, you'll don't know whether the information is actually viable, you still have to confirm it somehow, which might be difficult depending on what kind of information it is. In case of "The weapons are hidden at X" this is relatively easy to do, you send some agents to check that place out. In the case of "Who do we need to drone strike?" it's not as easy, because you can either kill them or not.