Thats the best question ever.
I don't know how many people buy a saddle and have no idea if it fits the horse or not. Myself included.
My grandparents, who are avid flea market goers, have bought me two cheap western leather saddles, one dark brown and slightly falling apart, and another in much better condition that is a yellow tan color.
I used the dark brown one on Siaga to get him used to the idea of a saddle on him, being as if he bucked it off and it went in the mud, no big deal. I don't think I've ever used the light tan one on him, being as I bought myself a nice red canvas and black leather one.
While Siaga has had it on plenty of times, I still don't know if it fits. I'm going to have to work up a sweat with him, so I can see if the saddle fits.
A lot of people don't know about sweat checking the horse or the saddle pad. When you take the saddle off and they've really worked up a good sweat, you can look on either the saddle pad or on the horses back, and the back is probably the best.
Check to make sure the sweat is evenly distributed on the area the saddle sits. Typically, but not always, there will be a dry line right over the spine and withers. If there is a patch surrounded by sweat that is dry, usually on the withers or in the front area, the saddle doesn't fit.
Why? Because if there are dry areas, then that means there's too much pressure there and its cutting of circulation of blood and also not letting the sweat glands work. This can cause discomfort for the horse, muscle atrophy, and skin problems.
So people, check your saddle sweat before you put that thing back on again and make sure it fits!