It really all depends on how much Montessori you are looking for. Schools can vary from following Montessori to a tee, to just sticking the name on a sign.
A few basics. There are 2 Montessori accrediting orgs in the US (not sure about Canada), AMS which is the Association of Montessori Schools, and AMI which is the Association Montessori International. Any school with an AMI accredidation is going to follow the Montessori principles within some fairly strict guidlines. The first Montessori that our son attended was AMI and we were very pleased with it. Since moving to PA, there are no AMI schools here, and we have not been as fond of the AMS schools. There is much more interpretation and other forms of educating going on.
As far as a classroom goes, you should be looking for a sense of purpose in each of the children. The idea behind Montessori is that providing a structured, well ordered environment, frees the child to concentrate on the work that they need to do to follow their internal compass. As such, the classroom should be neat, decorations should be simple, i.e classical paintings, vases with fresh flowers, etc. Children should be free to move about and find their own work, and should be doing this in a quite orderly way. The teacher, for the most part should be working with children in small groups or individually. She/he should speak to the children with a calm, quiet, respectful voice.
Some questions you may want to ask: How do they introduce a new child into the classroom?
Are children free to continue there work through group activities if they choose?
What kind of progress reports are kept, and relayed home?
You should also be looking to make sure they have materials covering each of the areas of Montessori, practical life, sensorial, math, language, science, social studies, music.
Hope this helps... Good luck!