I'd say look at the teacher's students. Do you want to be like them? If you see a good and harmonious sangha, and if they're practicing well and are good people with good hearts, then you have a cause for trust.
Tenzin PalmoFrom a Buddhist point of view, the first thing to help is yourself, to get your own mind together. And to really understand how to benefit beings, not just on the physical level, but on all levels. Then there are endless beings you can benefit.
Tenzin Palmo