1. Are they spinning their head?
If the other person starts spinning their head in answer to a direct question, they may be trying to hide something, or are outright lying.
2. Watch their shoulders, listen to their breathing and note their voice.
Three things a nervous person does are: raise their shoulders; speak quietly; and breathe heavily. Look for any of these signs. Their nervousness could indicate they are lying.
3. Are they standing rigidly still?
When someone is relaxed with you, they probably aren't hiding anything. So when someone is rather stiff and appears stressed, they are are trying to keep you at a certain distance. This is a sign they may be concealing the truth or are outright lying.
4. Are they having trouble expressing themselves?
They may not know what they are about to say, because they are making it all up now. They have no time to gather their thoughts and it comes across as very poor speech. Also they may simply repeat back your words to you. E.g. 'Are you really telling the truth?' 'Yes, I'm really telling the truth!'
5. Are they making an awful amount of eye contact?
If they are constantly trying to catch your eyes, it may well be because they are trying overly hard to convince you. They are probably doing this as overcompensation because they know they are lying.
6. Are they pursing and biting their lips?
When someone is not sure what they are about to say, they bite, purse their lips and swallow frequently. Their throat becomes dry through a lack of salivation. They may also be giving away the game with an obviously false smile.
7. Are they defending themselves a lot?
Watch their hands! If they are forever trying to defend a weak spot like they are defending themselves, they may have something to hide.
8. Are they touching their lips constantly?
Liars involuntarily give the game away by trying to cover their lips. This shows they don't really want to start lying, and their body seems to be fighting the urge to do so.
