I found the following things useful after touring a few nurseries/CMs:
How is their day structured? (ie how much is timetabled and how much is free play?)
What are the sleeping arrangements for children?
Explain how meal times/milk times operate.
What is the level of staff turnover/agencies used?
The reasons were: a lot of nurseries I saw had very very strict timetables so kids couldn't even have access to books without it being "reading time". The nursery I chose had quite a casual, homely approach, with some activities but access to things like books at all times.
The sleeping thing is because I found it (um, find it) very hard to get DS to sleep. Some nurseries put all the babies down at 12pm in cots in an upstairs room and then kept an eye on them with baby monitors. The nursery I chose basically tried to get the babies into a similar pattern for the sake of ease but rocked the babies who needed rocking, cuddled those who needed cuddling and stayed with them at all times. That sounded kinder to me.
The meal times thing is because at 9mo, DS would not take a bottle and was still breastfed all evening/night. The nursery had to help me out by getting DS to eat meals and yoghurts as he wouldn't take bottles. Some nurseries I saw acted like not having bottles would be a crime and a major issue. DS' nursery were relaxed about it and just saw it as a funny quirk in his personality that he hated bottles and milk so much.
Other things that mattered to me were staff turnover and agency workers (DS' nursery is family run and there have only one person has left due to ill health in the 18 months he has been there.) One nursery seemed to have entirely different staff on every occasion I visited.
Also the reaction of the kids to the staff. When the nursery manager at DS' nursery passed through the big kids room, they were asking her by name to play with them, which suggested to me that she was very hands on and warm.
She also asked me lots and lots about my baby - although he was only 10 weeks - and at other nurseries, they didn't even ask his name when I took a tour.
There are some negatives to DS' nursery - poor outside space and a few other things - but in general the impression I had when I left was "I don't want to leave my baby anywhere but I think they will try very hard to make his time there happy." That was enough for me.