Four thoughts here:
syllabus
references
alignment of interests
quality of equipment

1) Syllabus. Get a clear idea of exactly how every hour is going to be spent. If a school can't tell you this in advance, in detail, and document it with a syllabus, then they either have no plan, or no resources to implement the plan. Don't go there.

2) References. Talk to people in private. This is a tight business; you won't learn all that much of importance from the web. It's pretty clear who is who out here, and most people don't want to make enemies.

3) Make sure your interests are aligned with the school's. Ultimately, schools teach to make money. Some schools take big upfront payments, while others let you pay as you go, so you can still have some control over the process if things start to go sour.

4) Are the planes and FTD's up to date, functional, safe, and comfortable?