C.T.
Just observe the children. Are they engaged in their work? Do they interact at all? How do they interact with the teachers?
I looked at 6 Montessori schools before picking the one my son currently attends. I chose this one because the overall feel and flow of the classroom. Children were engaged, happy, and had some interaction with each other. I was lucky enough to observe one child act inappropriately (nothing bad, just speaking out of turn), so I got to watch how the teacher handled that.
It seems like many of your answers are fairly against Montessori, and its true that there is variation, which is why its so important that you observe the class. But, if you like what you see in the classroom (and I went twice before putting my son in) then it is probably a good school.
The school my son goes to is a bit of a fusion of Montessori and traditional. The teachers are all fully Montessori trained and accredited, but they add in direct teaching of some subjects (music, Spanish, etc.), and his teacher does guide him on what types of work to do. The children still have some choice, but they can't just do whatever they want. I really like it, because my son can work on what HE needs to work on ... not what the class in general needs.
Also, someone said Montessori doesn't allow play. I certainly would avoid any school that didn't allow play because play is developmentally necessary at this age. But, having said that, every Montessori school I have ever observed has had daily built-in play time.
There's no special commitment you need to make to go to a Montessori school. The only thing my school asks is that, at home, I try to let my child do things by himself to the extent that he can (e.g. dress himself).
Good luck with your decision!