I think you don't really need to watch more than 1 class before deciding to either participate in a trial class or look for a different school-- unless the first class you observed was somehow atypical, in which case I don't see the harm in watching a second (typical) class.
But I think you should be able to have at least 2 trial classes before needing to sign up.
When I first went to my school, I happened to show up on a testing day. So I watched that, and got a good overview of the different levels and different material at each level by watching everyone test, but I had no concept of what a typical class was. I went back the next night intending to just watch but I was invited to participate, so I did. However, I don't think anyone would have objected if I had said I would rather watch the class, since the 1st one I saw was not in any way a typical class.
What seems cool about showing up on test day is being able to see some of your future classmates humbled and stressed. This might take some nervousness away if a person were prone to this. But showing up on test day is not like an advertised tournament, of course. Usually only students know when that day is.
The trial classes do help. If nothing else you'll determine that the instructors breath just can't shake the smell of cigarettes and you can smell it both in standup and mat work!