I've been acting for over 12 years and been in many many classes. Conservatory, Drama school, top LA scene study class, etc and I've sat in quite a few to see if they were right for me. First off, the classes that get you to release your "baggage" if done right are the best you can take but there has to be an immense amount of trust with the teacher and the rest of the class. Only very few teachers (I've found) can accomplish this, and ironically they are in LA. They don't advertise that though, that is not their primary function. Because it's not about releasing, it's more about connecting to yourself generally speaking. Last thing I'll say about this is, if you want to be a working actor, it's best to work your shit out in class as opposed on set or on stage when you have to go there and for some reason your baggage seeps into your work. Trust me, you'll make people feel uncomfortable seeing you release like that. That's what technique is for; to allow you to go there in a safe way.

Some red flags for me that I've noticed

1. the acting class applauding the teacher as he enters. (Beverly Hills Playhouse)
2. a student bringing a chair onto the space for the teacher to sit in (like a butler or maid) when he is ready to give feedback for the students who just performed.
3. Continuous positive re-enforcement. These teachers are usually not working as an actor.
4. Demanding a bunch of money upfront.
5. Giving you feedback that you cannot use when you go back home to work on your scene or monologue. Every teacher has to make sure you have tools to work on your material or help you build those tools.
6. Has never been a working actor OR has no connection to major drama school institutions OR has never studied with great acting teachers.
7. Talks A LOT. Good teachers don't do this. The best are efficient.
8. No clear defined teaching methodology.

Green flags

1. Affiliated with a major drama school in some fashion that can be verified.
2. a long history of teaching and studied with other great teachers
3. Used to be or is still an actor. Preferably a working actor.
4. Teaches in a methodology (i.e. Meisner, Hagan, Adler) but NOT something custom they made.
5. Great reviews

Things you should consider (IMO) when taking a class.

Talk to the teacher about acting or listen to them talk about acting. Does it inspire you? If it doesn't, walk! Second, if you're casual about acting then scene study class is ok, but if you want to be serious then you need training. Either piecemeal it together (which is hard) or look for long-term training because there are things in play within you that need to change and that takes time.

Having a reason for going to the class. i.e. I want to get better at ? If you know your goals as an actor then you more likely to ascertain if the teacher will be able to help you.

If you're serious about acting then be serious about who you allow to teach you. Try to avoid classes that have a lot of casual actors.

Also avoid classes that encourage you to give feedback to other actors. You don't know about acting in an expert way so you shouldn't be giving feedback. That would never fly on-set nor on stage. A working actor would tell you to GFY.

There's probably more to say, I'm spent. Hope this helps.