Ask about her punctuality and also ask her referees (they may give you a different story!).
How many days of unplanned/unscheduled absence she had off in last job and how many of these days were because she was sick? -This also throws up days for this or that (both our previous nannies had quite a few of these sudden absences for reasons other than sickness as well as quite a few sick days).
Also, as already suggested, ask about medical conditions. Not sure exactly how it can be asked but I think it is important to know. Our last nanny suffered from a chronic, incurable medical condition that resulted in lots of time off and also could potentially have put my son at risk if she couldn't supervise him properly at times when the condition reared up without warning.
Our other nanny had an old knee injury that subsequently reared up again making it difficult for her to walk and requiring lots of time off. She never mentioned it to us when we asked about existing medical conditions and funnily enough she didn't mention it subsequently to a recruitment agency who asked her about previous medical conditions. For this reason, I would also ask referees (not sure how much they can reveal though) and I would consider preparing a medical questionnaire and asking if they can get their doctor to complete it. Not sure if there are rules on what you can ask on this though as we didn't do this with our past nannies but I wish I had. I have done this with au pairs though as it seems to be something that au pair agencies do so it seems acceptable. It has been revealing to see how many au pairs claim to have no medical problems until I ask them to get their doctor to complete a medical questionnaire for them (if they are on my shortlist and close to getting an offer). I have had quite a few au pairs suddenly tell me of complicated/rare/serious/chronic medical conditions they had 'forgotten' to mention before!
Finally, if they are live-out, I would also ask how they plan to travel to work and what alternatives they have if their usual method of transport fails - and check out their transport method (e.g. the train route) to see how frequent the service is.