What is the best way to respond when someone tries to cheat on you?

This could possibly depend on the situation at hand.  But what I will say is that I think the best way to respond to the individual at fault is not to go in with your guns blazing and raising some hellfire and brimstone, but to gently, as possible, ask them why they commited this act.  Depending on their response, there will probably be other questions that will come up but if you are going to defend yourself, do it in a calm and civil manner.