Next, you want to find out if IFR is taught as IFR is flown. Is autopilot use integrated into the training? Is the GPS navigator taught and used to the maximum extent possible? Does the school have a flight training device that can be used for familiarization with procedures before flying them in the airplane? Does the school have a relatively stable instructor group?
Once the school is vetted you have to become comfortable with an instructor. In the big and formal schools they have a system of checks and balances that ensures good progress through the course. When you are doing this at an FBO or local flight school it is more likely to be between you and one instructor, with maybe some checks along the way, and, at the end, the pilot examiner who will decide if the job was done well enough for him to give you the rating.
The great majority of the instructors at local flight schools are young people who are building flying time and sending resumes to airlines. On the surface this might seem like a bad deal for the trainee but that doesn't have to be the case. If these instructors are really interested in and enthusiastic about instrument flying, they can be good teachers of the art. There is some incentive for them here because most would much rather teach instrument than basic flying.
Because weather is such an important part of instrument flying you do want an instructor who has an abiding interest in helping you learn as much as possible about weather.
Weather and VFR flying is a pretty simple subject. VFR means visual, which means staying out of clouds and areas of poor visibility. There is a lot of black and white there. Weather for IFR flying is shades of gray. There we fly in the weather, not outside the weather, and weather wisdom is many times more important. It is actually critical, and learning as much as possible about weather is equally as important as the other elements of instrument flying. There are pilots who think they can be spoon-fed weather for IFR flights by an FSS, but these are the folks who get into weather trouble.
It would be nice if we could think that passing the FAA's knowledge test gets us where we need to be on weather. That is simply not the case. Weather for instrument flying is a much deeper subject and this is something you want to explore in depth with a potential instructor. Weather is one reason you want to find an instructor who will fly in actual conditions on training flights. If, in training, you do not plan and fly flights in actual weather, you won't have any way of knowing about the correlation between the information you get in a weather briefing and actual weather conditions. Remember, if you don't want to be a rated pilot who actually flies instruments, this isn't necessary. But if you want to be the real thing you have to plan on training in the real world.
A good way to examine the potential value to you of an instrument rating is to vicariously fly flights. Check the weather to see if you could fly a hypothetical flight in VFR conditions. If the answer is "no" then study the weather for a hypothetical IFR flight between the two points. A lot of weather elements affect IFR flights. There are clouds, ice, turbulence, precipitation, convection, fog, low ceilings, low visibilities and wind aloft to contend with. Only through study and actual flying can you put the proper weight on each of the elements that might affect your flying.
If all that makes it sound complex and demanding, good. According to the FAA, a pilot can get an instrument rating with 125 hours of flying time. That might be okay for a full-time student who wants to go on to become an airline first officer. But for a person who wants to fly single-pilot IFR in light airplanes, the FAA requirements are inadequate. If a pilot makes up his mind that he wants to learn how to stay alive while operating IFR it'll take a lot more time, a lot more effort and, yes, a lot more money. Is it worth it? Only if you plan to use it.