I don’t want to seem insensitive, but if you think he is, then he probably is. Human females seem to have a pretty good nose for these things.
However, let’s assume for a moment that you’re out of touch with your intuition, to the extent that you have to consult a simian overlord from another galaxy on whether he’s cheating or not.
One way you can be sure is to introduce him to the Trofmaldian Eye Slug. It really should be called a brain slug, because that’s where it does all its best work. What you need to do, is to drop the slug very close to his eye. The slug will do rest. It will crawl along his optic nerve to his brain, and wrap itself around his brain and allow you to determine if he’s telling you the truth or not.
Now this is important. While the slug is working its way into his brain (your boyfriend may scream a bit while this is happening) make sure that he’s looking at you. This way the brain slug will imprint on you, and allow you to (in addition to wringing the truth from him) control his every movement.
Once that little Trofmaldian nipper takes up residence in your beau’s brain, you should have your answer.
Oh, you should know that it will proceed to shut down his higher mental functions after a couple of months or so, and thereafter he’ll be a mindless zombie, doing whatever you say.
The perfect boyfriend!