Start talking about things along the lines of what you think he is hiding. If he tries to change the subject, makes less or no eye contact, is very quick and short with his speech, or changes the tone or confidence of his speech, he's probably lying or hiding something from you. If you accuse him of something (which you shouldn't do outright) and he answers abruptly and sharply, then he is also probably hiding or lying. Look for a confident, yet not aggressive "yes" or "no" answer. He shouldn't say it within the exact half-second you ask/accuse, and he should also be making eye contact with you.