Get them to be really clear about costs.
When you compare Nurseries, the cheapest hourly / session / daily / weekly rate or the most expensive - might not work out that way as it will depend on what is included and what isn't.
What happens about holidays - yours and theirs. Are they open all the time you will need them ? (No good booking somewhere that shuts for two weeks at Christmas, if you only get two days).
What about food ? Do you have to take your own ? Do they cook on the premises ? If so, do they have an actual cook or is it one of the room staff, on a rota ? Can you see the menus ? What meals would they have (I understand, for example some Nurseries give a 'Nursery tea' in the afternoon which I would have considered a pain as they wouldn't want to be eating their meal at home in the evening, but another parent might think 'great, I don't have to feed them'
Ask members of staff how long they have been there if you get chance. I take it as a positive if there isn't a high turnover of staff.
You can look up their OFSTED report on line, but it is interesting if they tell you things like that.
Ask about their special needs provision. Does the SENCo work in one of the rooms - is she able to get time released from ratio to support children in other rooms and attend meetings etc. What training has she had ?
I don't know how old your ds is, or what developmental stage he is at, but you might want to ask about toilet training and working together on that.
You could ask about settling in arrangements
You could ask what they expect you to provide - nappies ? wipes ? food ? Are they expected to keep wellies or rain suits there ?
I used to visit one Nursery once that had very high brow ideas and offered 'Spanish lessons' and 'Yoga' and 'Music Appreciation' ..... all of which they expected you to pay extra for, and I always wondered about what happened to the other children in the room, if 7 families paid for this nonsense and one didn't.
For yourself, think seriously about when you drop off and collect - parking, and what traffic you have to cross to get there.