As suggested in the link below, you can do it directly from the camera depending on the Canon model or possibly more easily by using
Backyard EOS software. Here is some reading on the former...
http://www.astropix.com/html/i_astro...esolution.html
It explains how to test the crop video to see if it is 1:1. Essentially take an image at Full res and compare it with a video frame taken from one of the camera's low res movie modes.
BTW I'm am Nikon
Best
JA