Originally Posted by RedRoan
Take a picture of after you ride in the saddle with the pad for over 2 hours...
Two hours may not be necessary. A good sweat is what you are trying to accomplish to see his back once the saddle and pad have been removed.
I just came back from a training ride on Boomer and after ~1/2 an hour we were both pretty well sweated up. I'm not convinced that the sweat marks are enough. Too wide an angle or too narrow may give decent marks with a lot of sweat but are still wrong. You want to just bring your horse to a light sweat for a better indicator. On the other hand if there are any dry
spots that is a no-no.
The other thing you want to check after a ride is any tenderness along his back or even welts. All those things are still only indicators. A good fitter or very knowledgeable person is the best - not a friend who "has been around horses forever". (read the first saying in my signature)
BTW, a buildup pad typically causes bridging.