There are many signs to spot. But the first question you need to ask is what is his reason for lying? Some things you may spot is he avoids eye contact or is acting weird. Try looking at micro expressions by Paul ekman.
I am sorry that you are having to go through this. I know that it is difficult, and I know just how scary it can be. Unfortunately, there is not much that you can do. You can try to ignore him, you...