This is a tough and important question with no easy answers. If there were some guaranteed system for definitively determining whether some group or teacher is going to lead people to liberation that would make life a lot easier!
One thing to look for in a group is, do the longtime members seem kind? Don't get distracted by whether they seem smart or businesslike or polished or well organized; none of those things necessarily matter. In America we want everything to be slick and new and polished but that attitude won't get you far with the dharma. But do look for senior students who are kind and generous.
Another trap to watch out for is, don't judge a sangha based on their adherence to contemporary liberal political correctness. While I personally think it's good to do things like recycle and drive a Prius and eat vegetarian and so on, the historical Buddha Shakyamuni did not do any of those things so it's not a valid basis for judgement of a sangha or set of teachings. In fact, I would say that anyone who is nasty and judgmental about such things is not very far along in dharma practice, because they are putting abstract principles ahead of actual kindness. And a sangha primarily composed of such people may have a poor quality of instruction overall.
Remember kindness, that's the short version of my answer.