As a simple test I just asked Erin, "Are you cheating on me?"
She laughed, looked up, and said, "Astrally?"
That's probably because of this thread:
Having Sex With Dream Characters (or Astral beings) Considered Cheating?
Hmmm... she never really answered my question, did she?
If your relationship is rooted in anything more than shallow infatuation, you might as well just use the direct approach with a question like, "Have you had sex with anyone else since we became a couple?" No point in being timid about it. If you can't be open and honest in your communication, then there's nothing to worry about because you don't yet have what you're afraid of losing.