I've been very careful about this, the poly guys want you to meet their wife, the cheaters either lie about being married or will freak out if you ask to meet the wife.
Still...It's easier to stand up for your principles in theory, harder in an actual situation with someone you're interested in. In my case, he was single, but was in a casual thing with another woman, they'd agreed that it was not exclusive/NSA, and he didn't tell her about me. The rational side of me said that I should tell him that I couldn't have any kind of sexual relationship with him until he'd told her about me and we'd sat down and talked and made sure everyone was comfy with it- but the emotional side was scared I'd lose him. I ended up deciding that he's capable of making his own ethical choices, and that I trusted him to do the right thing, and that I was only responsible for being honest with people I knew. Then he found out that she'd been having sex with another guy and directly lying about it, so now I don't have to worry about her at all.
I'm still disappointed in myself, but I think it turned out to have been the right choice for our relationship long-term. I trusted him to do the right thing, I was honest, I told him what would make me more comfortable, I didn't judge him.