I believe in body language to a point. Stuff like you can always spot liars if you are a master of body language is BS. There are massive numbers of people that are expert at lying both with their mouths and their bodies. Salesmen, strippers, lawyers, higher level business men, good gamblers, waitresses, bartenders, men that cheat and are good at getting away with it, women that cheat and are good at getting away with it, conmen/conwomen, sociopaths...and that's just for starters. Being deceitful unfortunately is advantageous in many professions. You can learn to be absolutely believable.
My fearless advice? If you like your friend more than as just a friend...you are either going to act on it...or you are not (yeah i know that is it's raining or it's sunny weather forecasting) Are you willing to act on it?