When interrogating people in LA Noire, the marker between truth and non-truth is pretty obvious. However, I can't tell how to distinguish between choosing Doubt and Lie. When does it make sense to choose one or the other?
In addition to Krazer's answer, I've also found that doubt doesn't necessarily mean "You're lying", it can mean "You aren't telling the whole truth, you're holding something back". If someone makes a very non-commital statement, even though it might technically be true, you have to choose between gentle coaxing for more info (Truth) or more pointed questioning (Doubt).
Also keep in mind that each question has one and only one correct answer. So if someone is lying, and you have evidence of that lie, choosing doubt will be considered a wrong answer. If someone is lying and you choose lie but don't choose the correct evidence, it is considered a wrong answer. You don't get partial credit for knowing that they were lying but choosing the wrong answer for it.