I phoned around the centres in my area (ones that were in a suitable location) and went to see the ones I was interested. You definitely get a feel for whether you like the place from the minute you walk in - cleanliness, variety of toys, are the carers friendly and approachable, do the kids playing look happy and healthy (in general) - right down to the smell of a place (it's generally going to smell to some extent of a cleaning product, particularly near the bathrooms, but the rest of the centre shouldn't smell offensive in any way).
One place I went to most of the things I wanted were there "on paper" - but when I went there I just got a bad vibe about the place.

Good luck - it's such a big thing trusting a centre and leaving your littlies in the care of someone else!