You know you've reached Atwater Market once you see all the flower tents on the outside.
Atwater Market was one of the two markets we visited in Montreal (the other being Jean Talon). It's pretty small compared to Jean Talon but has all the necessary produce for a local. There's even a Boulangerie Premiere Moisson inside for fresh breads and pastry.
As a tourist, I'd skip this market unless you're in the area. Jean Talon is the one you absolutely cannot miss on your visit.