So what I do is put the saddle on just behind the withers, no pads nothing and kinda jiggle it into place and if it doesn't stop moving it doesn't fit. If it settles into a spot then run your hand between the saddle and the horse if it's a really tight squeeze the tree is to small. Next make sure the withers clear the gullet and run your hand down the underside to make sure there is no bridging and the the rear of the saddle isn't sitting off the horse. Then put as much pressure as possible on the pommel and make sure the rear of the saddle doesn't lift up. Lastly make sure the billets/latigo lines up with the heart girth. If it's a pony make sure the saddle isn't sitting on their hips.
What I do is putting just the saddle (no pads or anything) on the horse, just behind the withers, and see if anywhere it moves. The saddle fits when it doesn't move around and doesn't lift the cantel when you put pressure on the pommel and vice versa. The final check is to see how it goes riding in it (and if the saddle fits the rider as well), so just use a thin saddle pad and walk, trot and canter.