First, ask yourself if your suspicions are based on anything that's changed with YOU.  If you're sure that you are being 100% the best partner you can be and don't feel she's reciprocating, consider what that means for your relationship.   You can talk about what is or isn't working out without accusing her of anything.  Maybe it's something completely off your radar.  If she can't have a heart to heart with you about the relationship, perhaps she isn't ready for the big time, so to speak, and you SHOULD re-evaluate things.  And maybe she'll tell you if she is seeing someone else if you can be the honest person you want HER to be.  But if she doesn't, and you just can't trust her, perhaps there's someone you can talk to about it.  Unless she's crazy and has a history of lying or something, there shouldn't be any reason why you wouldn't trust her, right? 

Hope that helps!