If you are not a jealous or suspicious person by nature and you are having suspicions, you may be correct.  We all have what we call intuition and usually your gut feeling is right.  The only time it is not is when you are letting past experiences influence your thinking (such as being cheated on before).  Keep in mind that there are many levels of cheating with the first usually being an emotional affair, so if you are correct in your feelings, you may be able to catch it before it turns physical. 

There are a couple of clues you can look for that I have found to be good in determining whether something is going on.

Has she withdrawn?  Basically has her attitude changed to where she doesn't care about things you used to do together or calling you and checking in, if she used to do so. 

Has she all of a sudden changed her appearance, either dressing up, changing her makeup or clothing choices, exercising or losing weight.

Does she hide her cell phone/email/etc from you or keep her phone close to her at all times?

 Does she seem to make excuses for going to the store or running other errands (if so she may be using these to see him or to call him)?