My DM was abused by her step-dad from the age of 13 (her DM knew but ignored it
) so it has always been on my radar.
When XH and I split up, I knew I didn't want to go the rest of my life without a significant other, so I did internet dating with the intention of meeting someone just for me, a boyfriend to see when the DCs weren't home, not a step-dad for them.
However, I met and fell in love with DP and have been with him 6 years, he is quite involved with my family, but he doesn't live with us.
You can pick up (if you're listening and watching carefully) whether someone is a boundary pusher, how they behave around their own/other DCs etc but in the end nobody can accurately predict who will end up abusing someone and you can drive yourself mad suspecting everyone, when in fact it is a tiny proportion of men who are paedophiles.
I'm very aware that statistically the most dangerous thing you can do for a child is to move an unrelated man into their home. But children are also abused by fathers, grandfathers, uncles, karate instructors etc and unless you are going to keep your children away from every man who isn't their bio dad for the rest of their life, you have to just put it into perspective, based on the age of your DCs (and how well they can communicate any boundary pushing etc), what the man is like towards you and others, whether he seems too keen to spend alone time with the DCs etc.
If something feels 'off' don't push it aside. If someone makes you (or your DCs) feel uncomfortable acknowledge that and get rid. We have a good sense for danger, its built into us, but our socialisation to be polite overrides our instincts.
There's a book called The Gift Of Fear by Gavin de Becker which talks about this. If you feel like your fear is holding you back from making good decisions about your life, maybe have a read and see if you can learn to trust your instincts more.