How to tell if your canvas is the right side to paint on???

i am hafl way through a reallly nice painting and have been struggling, trying to get a smooth look. After feeling a dozen other canvases, finally came to the conclusion that i am painting on the wrong side. Boy, it is really hard to tell unless you are actually painting. I hate to start over but it is sure rough and hard to get a nice smooth texture.

I have a whole roll of canvas. Wouldnt you think there would be some way to indicate which is the proper side???  They look so much alike and even feel the same. grrrrrrrrrrrr