There's probably very few things that are sadder than having to think about this situation. Infidelity, whether in a marriage or just a dating relationship is a hard thing to pin down and carries with it a lot of risks and issues. Those risks and issues include: 1. physical risk due to exposure to sexually transmitted disease, 2. emotional toll this "what if" question will have on you over time, 3. the risk to your relationship if you find out it's true, but also 4. the risk to your relationship if you accuse someone that is not really cheating on you.
I'm of the belief that the risks and issues of #1 and #2 outweigh the concerns of #3 and #4.
In order or tell if someone is cheating on you will probably want to avoid confronting them directly unless you feel absolutely sure. Instead, you might consider talking to your partner about how they are feeling. Something along the lines of: - How are you doing? You seem distant. - Are you happy with everything in your life? - Am I as supportive (or available, etc.) as you need me to be?
And if you feel like there is a noticeable change in your relationship, whether emotionally or sexually: - I feel like there's something between us emotionally lately, have you been feeling that way too? - Things have changed physically with us lately, why do you think that is?
All of these questions could, of course, lead you to things you would prefer not to know, but in this situation, the best thing to do might just be to get through it. What you learn will either help your relationship or will help you to grow personally for future relationships.