How can you determine if someone is lying by body language?

Contrary to popular belief there are no common signs to detecting deception through body language.

Body language is used to measure comfort levels, lying often causes the person to show signs of stress. Obviously if you observe someone looking stressed that doesn't mean they're lying

To determine if someone is lying you must understand how that person thinks, what is going on their lives, what is their opinion on the subject, what is their opinion of you etc. Body Language is just one of many factor you need to consider.