You may be able to see a copy of their latest Ofsted inspection online before you visit.
It's a long time since I did this but I would ask:
What are their prices and opening times?
What is the staff to child ratio? [Not sure what current thinking is, I'd expect 1:3 for babies, 1:5 for 2-yrs olds].
Does each child have a key person, what happens if that person is not there?
Do they supply nappies/wipes/bottles or would you need to bring your own?
Later on you would want to see copies of the menu, how and where food is prepared [on site? daily?] and what time meal times are.
Can you see a copy of their policies and procedures?
eg What if there is an accident, will they notify you at once, will you always be required to come and collect your child immediately?
Ask for a parents' handbook to take home.
What are their daily routines like?
Ask to see the toilet and nappy changing areas, are they clean and tidy?
What is their policy on potty training?
What would happen if you needed someone else to pick DD up who hadn't been to the nursery before?
HTH.