Just in case you are more of a beginner:

I would think that all of the sects that make up Buddhism consider their own teachings to be original and genuine, regardless of what others think. So if you don't yet have a working understanding of the various sects within Buddhism, that would be a great place to start to see if there is one that you are drawn to.

You didn't mention if these are lay groups or temple groups, but either way, trust your gut if something doesn't seem right such as someone being too interested in receiving a donation or membership fee from you too early on.

And even if everyone is super kind and wonderful, your own growing understanding of the Buddha's teachings may prompt you to change groups/sects in the future; so maybe don't get too attached. Good luck. :)

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.

When meeting other practitioners or teachers, always ask yourself - do I want to become like them? Do I want to look and behave like them in 5 years' time?

If they represent qualities you would like to master, stay with them. If you think you can trust them and learn something useful from them, stay with them until you feel you can't learn anything else. If they seem too stiff or wild to you, keep searching.

Take your time. Usually you can stay with a Buddhist group for a long time without any commitment: Go to the meditation and teaching sessions, see what kind of people there are and what they are doing, even for months and years, until you come to a conclusion.

(That is what I did. I stayed with one group for about four months, then with another one for three years, until finally taking refuge. It can be a slow process, and that is no problem.)

Study the Tipitaka, the Commentaries and see if what they teach is consistent with the books. Usually if someone is criticizing Abhidhamma in order to accommodate his/her views, it's a good indication that they have deviated from the original spirit of Buddhism. Regarding the Commentaries, a teacher might say "This is what the commentary says regarding this incident, but this is my personal view". That is reasonable. But if he degrades venerable Buddhagosa or other authors of the commentaries, that's a good indication to look for a new teacher.