Sometimes, it's difficult to guess whether he says the truth or lie… Actually you shouldn't guess! You may find it out by his movements.
Watch him attentively. If he closes the mouth by his hand, rubs, purses or bites his lips it means that he tries to keep his mouth shut. Literally.
In a childhood we often lied, having crossed two fingers. Remember?! So, this trick is subconsciously used in our adult life. Thus if your bf crosses his legs or arms or does the two things simultaneously, probably he says not everything or lies.
Eyes are a mirror of soul. To look into eyes and lie is the ability of few ones. Most people avoid visual contact when lie.
Another interesting index of lying is rubbing/ tugging at his ear or nose. When a person performs such movements, subconsciously he gives a signal "don't listen to me. I am lying". When he rubs his nose, he distracts attention from the lips, telling lie.
So, now you know how to make out whether he lies. However, I hope these tips