I deal with compulsive liars on a daily basis. No, I don't work in a mental institute but sometimes it feels like it lol! Some people are damn hard to read because they lie so often that they become good at it. Some people give it away with their actions like was mentioned. However, I find the best way to tell if someone is lying is to simply be critical of what people say. Not necessarily out load (it is effective tho) but think about it.
First thing I look for is how fluid the story is. You can kinda tell if it is being made up as the person goes along if it is choppy. Second thing is you remember minor details and match them up. Most people will make tiny errors that don't add up. Lead conversation about the topic and try to get them to back track or contradict themself. when they do: CALL THEM OUT! Calling out is the single funnest thing to do with liars hahahah
K, I'm done.