I'm kinda winging the interrogations. I read a book on body language (link below) a couple of years ago and it seems to be working out well for me so far. Tells are looking up and to the right, fidgeting, unable to look you in the eyes, facial tics, scratching, touching the face/nose/mouth, delayed response, etc.
I've managed to close the cases only missing one or two of the questions. My worst one was interrogating a bartender and I was 0 for 2. Bastard!