1 - He'll avert his eyes when responding. This is something most people do when having to come up with a lie on the spur of the moment. It's an involuntary reflex. If they look away to one side or the other, they're trying to avoid looking at you. If they look down, the odds are good they are trying to think up a response as quickly as possible.
2 - His breathing changes before he speaks. If you say something that might implicate your guy in a lie, if it's true, watch his breathing. He may hold his breath altogether, or his breathing rate may increase as his heart rate picks up. Both are signs that he might be lying.
3 - His mouth moves. This is another involuntary reflex that people do when mulling things over or are trying to think up a lie. Lip chewing, moving the lips from side to side, or pulling the lips in. All are signs of distress which may mean a lie is coming.
4 - His story doesn't add up. This is one of the more obvious clues that a guy is lying, but it bears noting. Sometimes when people are lying they have difficulty keeping straight just what sorts of things they've said. If you listen carefully to what he says, you might just find some discrepancies.
5 - He clears his throat. Yet another involuntary gesture that people do when trying to put words and thoughts together and are finding it difficult due to having to make something u quick.
6 - He takes longer than normal to answer. Normally, when a person relates something that happened or the events or ideas that led up to things, they simply dive in. On the other hand, if someone has to change history so to speak, it takes a second or two to put the story together. Of course this is only true if the person is put on the spot. If they make up their lie beforehand, it's likely to pop right out with nary a lag in time.
7 - He's sweating. If there isn't any apparent reason for him to be sweating, hot weather say, or recent physical exertion, then you have to ask yourself, why is your man sweating? Is he nervous? Anxious? If so, why?
8 - He tries to change the subject. Another thing that people do, especially guys, is to try to change the subject if the topic is hitting on something he'd prefer to be kept hidden. Granted, he may simply be uncomfortable with the topic, but when consider this type of action along with others mentioned here, it can help build your case.
9 - He puts it back on you. Something men do a lot when cornered, or caught in a lie, is try to reverse the situation and put the onus back on you to prove what you're saying, or to defend yourself from some other charge. This is pure manipulation and is usually a very good sign that your man is not being up front with you.
10 - He sets his jaw. This is something men do more than women also. They set their jaw tight, or move their head a certain way so as to put forth a front that tells you they are digging in; which means they've told you their story, and now they're going to stick to it no matter what. While it's not proof of a lie, it is a sure sign that they are going to defend what they've said to their bitter end.
These ten pretty good signs on how to tell if a guy is lying are intended for women who are with a man and are wondering if he's being honest with them. If you are such a woman, I hope these signs help. Good luck in getting to the bottom of things.