If you think something is going on then you are probally right.  

Confronting one about infidelity doesn't usually lead to the truth.  See what you can find out.  Do a little of your own investigating.  You would be surprised to hear what one's friends may tell you.  Even his closest friends.  No one likes to see another person hurting or being done wrong.

Good luck...

If it is your gut that you feel he is cheating, then he may be. Is there something that you are hiding from him which could cause these guilty thoughts? I do not mean you are cheating.. but think about being happy when another smiles at you, how does that make you feel? Or that conversation that made you wonder... hmmm.. maybe I still do have it? I mean that type of guilt that makes the mind question the other. Then when I say he may be cheating.. it could be a form of having a friendship you do not know about.. more than just a co-worker... sometimes it could be a simple form of the larger feeling. Hope I didnt lose you there, I am still working on turning my thoughts into words for others.

So.. what I suggest, make time to go out together. Ask yourself, what is it that you feel.. when in doubt.. ask. Best Wishes!

** these are my thoughts and not to be taken as relationship saving tools... ;)