After 10 years of experience as an equine massage therapist I have seen hundreds of horses living in pain due to poorly fitted saddles. In this great article from thehorse.com I found 6 easy steps that any one can use to help understand how a saddle fits.
Step 1: Is the saddle level? Or, is the deepest point of the seat horizontal to the ground? This doesn’t necessarily mean the pommel and cantle will be even to each other, however. If your saddle has an uphill appearance, the tree is likely too narrow for your horse. Conversely, a downhill appearance is indicative of a too-wide tree.
Step 2: Does your saddle rock forward and back? Put one hand on the pommel and another on the cantle, applying alternating pressure to see how much the saddle moves forward and back. Ideally, it should remain fairly stable. “If the saddle rocks forward and back it is a sign the tree is too wide or the panels are too curved,” he said. “If this is the case (tree is too wide) there is usually a lack of contact at the rear of the saddle.”
Step 3: Is there adequate pommel clearance? The pommel shouldn’t rest (or come near to resting) on top of the horse’s withers. Although the amount of space between the withers and the pommel will vary based on saddle type and horse conformation, there should be clearance between the pommel and the top of the wither when the rider is in the saddle.
Step 4: Is there consistent contact between the withers and the tree? While the pommel shouldn’t contact the top of the withers, the tree should contact the sides of the horse’s withers. “Ideally, the contact should be over a broad surface area without areas of focal pressure,” Slide your hand between the withers and the tree and feel for 4 to 5 inches of smooth and consistent contact. If most of the contact is located near the top, the saddle is too wide. If most of the contact is farther down, it’s likely too narrow.
Step 5: Is there consistent contact between the panel and the back? Slide your hand between the panel of the saddle and the horse’s back to check for pressure points, bulges in the panel, and areas without contact between the two. This can also be done visually before the saddle is placed on the horses back
Step 6: Does the horse’s back look healthy? “If the horse has been ridden regularly and recently in the saddle being evaluated, examination of the back for patterns of pain, swelling, rubbed hair, and coat and skin abnormalities is helpful,” feel the horse’s back—including the scapula and spinous processes—to look for signs of pain, such as moving away from the pressure or excessive muscle contraction