Talk to the staff. All the staff! When you're shown round they might try to hide duff staff at the back of the room. Ask basic stuff like why do you like working with children?
Look at how clean things are. There should be some mess! I've worked in nurseries that didn't let the children do much messy stuff because it meant cleaning up which was work for them.
Is there a selection of things for the children to play with, and are they allowed to play with it? 2 of my nurseries had computers they showed off to parents but were actually not usable.
Try to taste lunch.
What ratios of staff to children are there. 0-2 years should be 1:3, 2-3 years 1:4 and above 3 years 1:8. They are Minimums! A good nursery would always try to have more staff than the minimum.
Do the children go outside? Really? Lots of staff can't be bothered.
Are the staff happy?
In the older children's room, ask them to explain how they plan to provide activities to promote all areas of development. Can they tell you without umming and erring?
After they tell you that they do such and such, ask why? They might do the right things, but do they do them just because they have to, or do they know why they do them?
Ask the staff how long they've been there. High turnover means disruption for children and the nursery isn't a happy place to work in.
How many supply staff are there? It's good if it's to cover illness but loads of supply staff means they can't get people to take jobs there full time, because it's crap.
I could go on for ages but happy, good, caring, friendly staff who know what they're talking about will mean a good nursery.