I know you, bofish, enjoy casual hook ups like me. Quite spontaneous ones. It's very hard to have a spontaneous hook ups if you are going to insist on verifying with their partner whether they are allowed to bang you. You need to decide whether fucking a cheater is so horrific that you will sacrifice casual sex in case you stumble across one.
What I decided for myself was that if someone took my fancy and they didn't scream cheater at me, I would bang them/date them. When and if I discovered they are a cheater, I would abandon all relations. It's far too much hard work to verify everyone's story and I believe in trusting people until they prove otherwise. I don't feel stupid when I'm let down. It's usually them who did something wrong, not me.
Cheaters do approach me and are usually honest about their situation. Some people just make me feel queasy. They may present as poly, but maybe what puts me off is the fact that they are lying about that. I don't know. I don't confront them, I just don't reply or retaliate to their advances.