Hire Magnum P.I., and he'll uncover a whole mystery that reaches the highest ranks of the fucking Mafia.
No, but seriously, look out for behavioral changes, name mixups, suspicious calls and texts, and that kind of stuff. Also, try coming home at times when your significant other least expects it, and maybe you'll find what he who is master of his own reality said, then brutally murder them both with an axe. If all else fails, set up some hidden cameras around your home. If it's true, you might get some good happy time material out of it.
This is one of those topics that I just can't resist making jokes about, so I have to cross out some of my post.