Spotting an anxious face
Obviously this is not something you would do to someone who actually had anxiety, but to someone who is only anxious because they cheated on you it’s not the nastiest thing in the world. You can look at them as if you know something, but obviously keep quiet. They will start to feel nervous and their mind will get the better of them. Eventually they might start doing really weird stuff because they feel on edge. Just don’t let them know you are trying to play games with them.
Check your own computer
I would never tell you to install software on someone’s computer, but if you share a computer with someone you might think it’s OK. There’s a special type called keylogging software and you can look at it once someone has left the house to see what they were doing when you weren’t in. If someone is speaking to someone else it won’t take you long to find out. You can’t beat physical proof.