We just had a great saddle fitter do a clinic and I'm trying to remember everything she said to do. There were 6 things to check. 1. Make sure the saddle is in the correct place, behind the shoulder 2. Take your flat hand and run it from the pommel, down the underside of the flap and see if any spot appears tighter or not to fit your horse's shoulders/withers. 3. Check and make sure you can fit at least 3 fingers between pommel and withers 4. Put your right hand under the pommel section of the saddle (bars), push down with your left hand on the seat and run your right hand from front to back, seeing if the pressure is even or if there are any gaps or pressure points. 5. Look from the back down the channel and see if you can see all the way to the front and if the channel appears wide enough to fit your horse's spine. 6. From the side, make sure the pommel of the saddle doesn't go past the last rib (where the hair from the stomach and the hair from the flanks meet and go downward. Every time we have a saddle fitter, I learn something new!
Hope this helps!