It is different with everybody. There are certain facial cues. Watch the eyes. You can almost always tell by the eyes.
It isn't just eyes that won't focus on you, it's eyes that stay too focused on you. If they won't make eye contact or won't break eye contact those are some flags. Naturally people's eyes move during conversation but will also attend to the other members of the conversation for short periods of time.
Another sign is the way which they tell lie. Does it come out quick and jumbled? Does it sound rehearsed? Did they add details later in an offhand manner? Are they recounting insignificant details? All little signs.