If u watch the news, u'll find that our kids are really vulnerable. It seems like no matter what we do as parents, there is always a loop hole for your child to get exposed.
Anyhow, there are loads of very good babysitters and I believe the best recommendations come from the kids themselves and not anyone else. So after the sitter leaves, just really talk and pay attention to what they say.
Now, I don't see anything wrong with keeping tabs with whatever is going on with your kids during your absence. If I had spy gadgets i would use them for the first couple of days and I'm sure I would be able to tell if the sitter is a good fit based on her interactions with the kids while I'm away. I think its not that difficult to tell the difference between a bad siiter and a good one.