The only way to be sure whether someone is cheating is to walk into the bedroom and find him in flagrante delicto. The problem with asking him if he is cheating on you is that someone who cheats will likely have no scruples lying. But fortunately, by watching his face and eyes, you can pick up the subtle signs that suggest prevarication. No one sign is enough to prove lies, so study them all and see which show up during the conversation.

Before you dive in to the conversation about cheating, talk for a time about something else. This part of the conversation is just to establish a baseline, getting a take on how he usually acts and what facial gestures he uses when he is telling the truth.

Talk about the weather, mutual friends, the new movie coming out this week or his favorite sports team and watch him like a hawk. Note any tics in his facial expressions and particularly note which ways his eyes move. Eye movement is associated with accessing memory information – usually the eyes point up and to the left for right-handed people or up and to the right for left-handed people when they are actually considering a memory. As you hold this part of the conversation, confirm that this general rule is true for him.

The next step of the conversation is to bring up cheating. Ask him pointed questions and watch him carefully. Do his eyes start moving in the opposite direction as before? That likely means he is not accessing facts but pressing his imagination to come up with a persuasive cover story.

Other signs of prevaricating can also be seen by eye movement. Do his eyes dart back and forth? Does he blink rapidly five or six times, or blink, then hold the eyes closed longer than a normal blink? All are signs that he is uncomfortable with the question and may be lying. If he smiles but the eyes don't crinkle, that's a fake smile and may also indicate lying.

Touching or scratching the face can also mean lying. Some people blush just after lying, while others shake their head as if they were saying no. Excessive sweating either means he's very ill or very nervous, usually an indication of lying. And as the body fluid comes out in sweat, it leaves his throat dry. That can cause throat clearing or hard swallowing, both of which also indicate that lies are being told.

Any one or even two of these signs shouldn't make you sure he is lying about cheating. But if you see many of them, the odds are good that you are in a relationship with a liar and a cheat.