I think the simplest way to avoid getting/staying involved with a cheater is to reflect on the fact that they're lying to the person they're supposed to love most in the world, so of course they're going to lie to you too, sooner or later. What makes you so much more special than the person that they've committed their life to, that they'd lie to that person but never to you? Nothing, that's what.
Sure, they may seem honest and open in extreme. But you're dealing with someone who is extremely well versed in hiding things, as they do it every single day. Every single day they look their partner in the face and act like everything's fine. Can you trust a habitual liar to, say, ve honesty with you about their STI status? I wouldn't count on it. So why put yourself in harm's way?
Until such time as a cheater has come clean about that behavior to the person they were hurting, sworn it off, and lived a lie-free life for at least some amount of time, I wouldn't consider them a safe person to get involved with.
Me, 30ish bi female, been doing solo poly for roughly 5 years. Gia, Clay, and Pike, my partners. Davis, ex/friend/"it's complicated." Eric, Gia's husband. Bee, Gia and Eric's toddler.