We have a nanny, and she was the first, and only person that we intereviewed. She's AMAZING and she loves my son as much as we do. (And most importantly, he loves her!)
She teaches him so much and constantly has him out with other kids playing.
She came to us as a resferral. We had friends of the family that have her sister as a nanny, and she told them that our nanny was looking for a job (she was moving here form Belize). We knew that those friends were VERY anal about childcare, so we wanted to interview her. We just knew as soon as we met with her. Had *that* feeling.
Depending on where you go to find your childcare, the others in that area will tell you everything you need to know. For my nanny, all the other SAH moms in our little housing area see her with Mason and RAVE about her. Literally, neighbors that I don't know at all have come up and commented to me about her. The also tell me horror stories about other nannies, so that word spreads! So start with referrals, interviews, and then ask around a lot once you settle on someone. It may seem cheesy, but the nanny-cam is a great way to give you piece of mind if you have nothing else.
We also had ours come a few times a week while I was still home on leave. That way I could get to know her, and she could get to know Mason. It made me feel a lot better about leaving him with her when I had a better feeling of how she'd be with him. Sort of a test-drive, if you will.
If you'd like, I'd be happy to share our interview questions and nanny-agreement that we used with ours.