Phrase the question more unconventionally. The unusual wording is able to "confuse" anyone, even the most experienced liar. In order to bring the liar to clean water, it is necessary to behave confidently. We can even rehearse the intonation and facial expression in front of the mirror. You should put down a man with conviction and a desire to hear only the truth. Make up the questions so that answer was a huge amount. People are tired to choose and in the end he will have to speak the truth.
Put a person in a position where he just cannot lie. Select the time and situation in which people would not expect a "trick". Thoroughly prepare. Choose a time when you will be alone and create as comfortable as possible in psychological terms the situation. To lull the attention of human - friendly, and then suddenly ask a question that has long want to ask him. Watch for a reaction. The liar can pretend to be the movement of the eyes (take them aside, down), touch the face, the nose, rubbing the forehead, etc. If you have noticed similar symptoms - continue the "attack." Confidently repeat your question, don't give the man time to recover. You have not the most pleasant task, but a liar you need to drive into the corner that he just did not have time to come up with a new excuse or lie.
Use the "forbidden" techniques - blackmail or a threat. Of course, this method can only be used in extreme cases when other methods have been exhausted. Of course, you won't realize your threats, but because the liar are not aware. Do not have to openly threaten, just suggest that you determine the well-being of the liar. Fear can provoke him to speak the truth.