One easy way to do this after working a horse hard enough to make it sweat, is to take photos of the sweat patches as soon as you take the saddle off, and check whether there are any asymmetrical areas.
This is after removing Daisy’s Butet, taken from the offside, so that her withers are to the right of the photo. You can see that the line down the gullet is pretty even. I didn’t see anything to worry me here.
With some saddles you will see areas over the scapulae which are obviously not in contact with the saddle panels, which means that there is more pressure elsewhere – obviously the more even the sweat patches, and the bigger the area (within reason), the better for the horse.