I told by my grandma that the best part of the body that is good at lying is the mouth, but the part that tells the truth is the eyes, the best thing to do is to look at the persons eyes and not the mouth because its the best place to extract the truth.

the eyes moves rapidly than usual and eye lid closes, these are the best sign to show that lie is being delivered.

this is just a clue so dont depend on it too much, sometimes it doesnt work