First, you need to know as much information as possible about the nanny. The more information you know about her the easier it is to find even more data.
Here are some ideas: Ask her to fill out an application in which details like her date and location of birth, previous locations lived, names, addresses and phone numbers of relatives, schools attended and communities in which she participated will be listed.
Why do this? Because in step 2 we are going to start searching using some of the world's best services...
I don't believe in those 'specialized nanny background search services' because I have witnesses they are like any other people search service.
So the best thing we should use here is a great and cheap background check service. Be wary there are lots of scams out there that promise something like full access to all kinds of records for just $20 or so. If something is too good to be true, it probably is.
I recommend you use USSearch (link below in the resources section.) So go to their website where you can search by name, address, phone or social security number.
Once you start searching you'll get PLENTY of results. This is why I asked you in step 1 to get as much information as you can about your nanny! We need it to distinguish her from the other search results.
You found something that looks like her? Great. USSearch offers cheap background report service that I think is worth. So I recommend you invest a little bit of money into it.
Check if your nanny has a social networking profile (you can use Wink.com to search through multiple social networking profiles like Facebook, Hi5, MySpace and many others.) People usually leave lots of information online so you might find something handy for further research.