The NCAC website will have some really helpful info on what to look for and what questions to ask.
I made a out a big list and took it with me, but TBH I knew about 2 minutes after walking into the centre whether I liked it or not.
In the end what impressed me about DS' daycare was

-the director was very approachable, she actually took my list of q's adnd adressed them all individually, I was there for at least 45mins with her looking at the rooms and children etc.
There was a good mix of staff ages groups and a high staff retention rate.
The relationships I observed between the staff and the children were great.
- the centre was well maintained. It wasnt new and shiny but it was looked after
- they employ a full time chef and prepare everything fresh.
- they have behavioural management techniques in place and work with parents to help childrens behavoural issues resolve (This was very handy when DS was biting..)