Ask yourself if you have had these suspicions before.  Are you insecure? Has your lovemaking changed?  Has her appearance changed?  Is she more concerned about her appearance?  Has she lost weight?  Does she seem stronger and have less patience with you?  Does she avoid the crowd you hang with? She may not want to do the things you two routinely do.  Is she dissatisfied with you lovemaking and is catching a little on the side with no other attachments.  My number one is communication...and just ask.  Nothing like being to the point and getting it over with ..or maybe she'll have an explanation.  Are you happy in the relationship?

Hope this helps.