The way I got a handle on this in my region was going to competitions and watching the pros with their pro-am students for the styles I was interested in. You get to know names, a lot of programs include studio names or locations, you can see for yourself how they dance, how their students dance, and how they treat their students on and off the floor.
So for instance, I was able to find the style specialist a couple of hours away that was awesome in all the aspects that mattered to me. He made his students look awesome on the floor for their level (e.g. got his dedicated students to open and placing really well, but also knew how to make his newcomer and bronze C ladies look really well presented). I could tell he taught and danced great technique by watching him and his students. I could see he treated all his students with respect. I could even see if I liked his styling choices for the style in question. I could also tell over time that he was well respected by other pros. Likewise, with a different pro, I could see that his students placed well, but he was really not a nice guy, and I personally wouldn't pay to be treated the way he treated his students, regardless of what he might do for my dancing. You can only get a fraction of this info from looking at placements of the students of the pros at comps. Just looking at placements, you can't tell the dedicated student from the one lesson a week does it for fun student.
Recommendations are a great place to start. But do some first hand observations when you get a chance.