Is she a nanny, I am making that assumption if she is coming to see you and therefore will be caring for children in your home.
need to see crb check, qualifications, any written references (also ring 2-3 of these for verbal references).
Nannies should bring a portfolio of relevant information with them.
think up a couple of scenarios to ask for example if you have a fussy eater and usually you can get them to eat with encouragement maybe say X wont eat the lunch youve prepared and says he will tell mummy you are mean to him as she lets him eat what he wants.
what would she do in an emergency?
does she need to drive? if so are you concerned if she has points on liscense, if so how many is acceptable?
Ask her for a rough idea of how she would plan a day - most nannies will have researched whats in the area activity wise before coming to interview.
can she cook, ie prepare meals not just heat up prepared food you have made unless thats what you want.