How we change what others think, feel, believe and do
Just ask the question for which you want an answer.
Phrase the question such that it is simple and clear. Ask in a pleasant, but neutral voice. Show neither threat nor anxiety.
Note how quickly they respond. If there is a slight delay, they may have been considering their response and hence lying. Sometimes a too-short delay can show they had an answer ready (not a good a sign, as it indicates they are prepared for interrogation).
Use this method first, before using any other method, unless you have reason to be more surreptitious in your approach.
You can also throw in a direct question at informal points where their guard is down, for example when giving them a cigarette or just as they are leaving.
What is the name of your captain?
How long have you been in this country?
Do you speak Italian?
The average person, when asked a straight question, will give a straight answer without thinking too hard about whether or not they should give the answer. We are largely brought up to comply with the requests of authority figures, including answering of questions. There is also a social norm that you always answer questions, even from strangers.
Sometimes subjects do not know the importance to you of questions that may seem innocent. Sometimes they will think that answering simple questions will keep you away from the more difficult questions. Sometimes they want to show that they are harmless. Sometimes they are expecting more aggressive methods and a simple question catches them unawares, especially when they are not expecting one.
Whatever the reason, a surprising number of subjects will truthfully answer a simple and direct question, particularly if they have not been trained in anti-interrogation techniques.