If he gets jealous when you are around men that will show he is guilty of what he is doing. Even if it is in his mind and he is not doing it physically, he will still have insecurities and he will automatically throw up a guard against you. If he is not doing anything wrong, you will know if he is or he isn't.

do you have access to his computer?do you really want to see what he is up to?2 easy hints to gain access....1-control h pressed at the same time gives you his history 2- at the start button there is a box for search,press the letter t (after search everything)and it gives you the temporary files from the internet. Usually though,and I hate to admit this if your gut says he is than if you look you WILL find something.

email him from a new email maybe a myspace or something the like and hit on him and see if he takes the bait. but if you have a picture make sure shes not prettier than you so he has an excuse. and if he passes or not never let him know it was you.

hire spies to watch him everyday and night. Find a way to tap his phone calls, break into his email. :)