You can pay attention to how they act while answering your questions, but sometimes can be difficult if they're a particularly animated character, like the bar tender in the hit-and-run or the perp in the last patrol case. Best thing to do is wait a few seconds after the person answers your question. If the person:
Continues to look straight at you, they're telling the truth.
Looks away, usually to their right, there's doubt.
Looks away, generally to their left, they're lying.
The difference between doubt and lying though, is evidence. If you call a person out for lying, you have to have proof to back it up! Doubt is basically calling someone out, maybe not necessarily in a lie, but perhaps not telling the whole truth.
Well, this is what's worked for me thus far anyways. Just remember to keep track of your evidence so you can catch those lies.
Just completed the Golden Butterfly case and the Silk Stocking case, loving it so far. Can't help but wonder if I'll be revisiting these, the Golden Butterfly was very similar to the case before it.