the fact that you need to ask says you have your own doubts ... I think you should follow your gut...
The biggest sign to pay attention to is your INTUITION. It is never wrong, though many have ignored it, to their dismay.
When he is cheating, starts dressing more smarter, reeks of aftershave etc..
When you find that he shaved his balls (all of a sudden).
having a voter registration card is the first tip off
Your strings are making noises? (Sorry-couldn't resist)
There's a lot of smirking.