It depends of course how good they are at lying but body language is one sign - not making good eye contact when a particular subject is discussed - avoidance. Or making too much eye contact - trying to brazen it out.
Depending on how their body usually reacts to difficult situations - do they flush or go pale - when certain things are talked about? There could be other signs such as not willing to discuss certain things. Or being overly sensitive when a certain subject/person is spoken about. And watch for the 'back-lash' one. This is when a person who is lying tries to off-set their guilt by becoming angry and accusing you of such things as 'jealousy', 'paranoia' and provided you know for a fact that you are not just jealous or paranoid etc. Then this aggresive behaviour may indicate a big 'guilt trip' about something.