It's mostly been said, but:
1. Take a look round the club first. Do the folks talk to you and try to understand what you're there for, or do they ignore you and continue swapping war stories round the bar?
2. Do the aircraft look clean and well cared for? Old-looking aircraft aren't necessarily a bad sign, but personally I prefer clean, well maintained and airworthy aircraft.
3. Will you always have the same instructor? How experienced is he/she? Meet said instructor and see if you think you'd get on well together.
4. Do they tell you at the outset how much ground school you will get/will need?
5. How much are landing fees? Are they included? They can cost you more than the flying if you learn at some SE airfields.
6. Do they offer "block discounts" (good, if you can pay for say 10 hours at a time), or ask for thousands upfront (walk away!)?
7. Talk to some students already there - and ask their view of the place. The good schools might even arrange for you to do that.
8. If in doubt, go elsewhere.