There are some good-read, government-published guidelines from places like Canada and the UK, where professional childcare is a little more widely available, but most suggestions--observe the carer/set-up all day; interview parents, not just the carer or the daycare center chief; watch closely for reactions and unspoken nuances to your questions--presume a lot of self-awareness on the parent's (i.e., your) part.
For example, "Ideally, you'd want the daycare to treat your child(ren) as close as possible to the way you treat them." But face facts, if you're really honest about occasionally plopping the kid down in front of The Wiggles so you can safely ignore him for a while, do you want a carer to treat him "as close as possible" to this? Or better?
At the park this morning, the kid showed up with a bucket and some digging tools, enough to go around (which they immediately did). But there was one nanny who would spirit her kid away immediately any time another kid started playing near him. And another kid, who was hoarding several shovels and animals--which attracted a steady stream of would-be sharers--screamed whenever anyone got near them, and the nanny would just say, "No, those are Chloe's and she doesn't want to share." Never mind that Chloe is about 16 months old.
From these [admittedly incomplete] breaches of the playground's social contracts, I had to wonder if these kids' parents knew that their caregivers were training their kids not to share and not to play with anyone. And how would you know beforehand?
Evaluating child care providers [ask.metafilter.com]