I'm not a trained interrogator or a trial lawyer, but I do have a very good track record as a reporter, and that involves evaluating people and their likelihood of lying.
You might be interested in taking one of Paul Ekman's deception detection training programs and learning his Facial Action Coding System (FACS).
The Facial Action Coding System (FACS) is an internationally recognized, sophisticated research tool that precisely measures the entire spectrum of human facial expressions. FACS has elucidated the physiological presence of emotion with very high levels of reliability. In fact, it has predicted patterns related to deception at about 80% accuracy.
Created in the 1970s by psychologists Paul Ekman and Wallace V. Friesen FACS provides a comprehensive taxonomy of human facial expressions. FACS remains the most widely used and acclaimed method for coding the minutest movements of the human face.
Essentially, the system dissects observed expressions by determining how facial muscle contractions alter appearance. Each movement is categorized into specific Action Units (AUs), which represent the contraction or relaxation of one or more muscles. All facial expressions can be decomposed into their constituent AUs and described by duration, intensity, and asymmetry. Trained experts examine patterns in the changing nature of facial appearance including: movement, changes in shape and location of the features, and the gathering, pouching, bulging and wrinkling of the skin. Understanding the coordination between action units and certain expressions illuminates the implications of human body language and non-verbal behavior.
Once you learn the nuances of the Facial Action Coding System and develop the vocabulary to analyze what you're seeing, you'll never look at people the same way again. Seriously, no matter how good you are at detecting bullshit, Ekman's research will put you at a real advantage.
For those of you who don't have an employer to foot the bill? As usual, Russian libraries and torrents deliver. Ekman has written some popular books, but they're no substitute for taking the time to absorb his academic work. If RAND, CIA and DoD take it seriously, so should you.
And as long as we're talking about detecting bullshit, here's a link to Barton Whaley's magnificent magnum opus which I can't recommend highly enough:
DETECTING DECEPTION: A BIBLIOGRAPHY OF COUNTERDECEPTION ACROSS TIME, CULTURES, AND DISCIPLINES
Bonus track: The Platters - Every Little Movement (Has A Meaning All Its Own) (1962) lol