"The Lord searches all hearts and understands all the intent of the thoughts."
Some signs of lying are fidgeting, lack of eye contact and noticable pauses in speech. However even with these clues it is still hard to tell if one is lying. There is one sure way to know tho, seek God's council, ask Him to show you the truth in the matter at hand. He will always show you the true way. Also the inner Holy Spirit will help us discern what is truth and what is a lie. So remember if you aren't sure of the truth in a situation, wisdom from above will help us decide who to trust!