All the above. ... And most of all trust your own gut instinct. It will tell you if it's the right place for your family.
Your children will also be trusting their gut too so keep an eye on how they respond to the centre and see how staff respond to you and your children too.
It's important that staff are happy and relaxed because that influences how your children's day will be.
I'd also ask about staff turnover, family involvement ad flexibility in responding to children's needs and interests.