We use our gut. Since DH works so much and isn't around to meet others, he trusts my opinion on whom our kiddos spend time with, both our biodaughter and my DSDs. A few months ago, our next door neighbor stopped by and asked if our girls (6 and 4) could go to the park with her and her son and niece. I said yes, and DH was just surprised when he got home and the girls weren't there.
He was slightly concerned at first, but thought about it a second and realized I would never put our kids in danger.
Also, even though he's never met any of my mom friends, he's totally willing to ask one of them to babysit should we ever get to go on a date again.
We just try and go with what feels right in the situation. Hope that helped at least a little!