He may be angry at being falsely accused, or having his honor questioned, but he does not feel guilty. He mirrors your posture. He talks expansively with his hands, starting the gesture before the words. He is relaxed and his smile engages other facial muscles. He points to himself and places his open hand on his chest. He is not afraid of close scrutiny.
Liars often say honestly, believe me, or I'm telling the truth. He may be smiling, but inside he is sweating. His brain races to come up with the details it lacks in answer to your questions. It is said that a liar doesn't memorize the story backwards, so asking him to repeat the information regressively trips him up.
If Dick asks questions like, “So, when did you go to Starbucks?" Jane has to think hard about what she just made up. Did she say she stopped at Starbucks before or after department store? If your teen gives you a list, ask him to repeat it backwards. I bet he can't.
I hope you've enjoyed our lessons on body language. Now, go revise! If you want more hints on how check out Story Building Blocks III: The Revision Layers.