One of the very best ways to determine if your saddle fits is to stand to the side of the horse and take a good look at its balance on the horse's back. Here are some common problems which will help you determine a good fit.

REMEMBER: saddle fitting is only partly about horse comfort. It is also about your comfort, and about making it possible for you to ride correctly.

Have you ever felt like you can't get your legs underneath you when you're riding? Have you felt like you're sitting in a chair seat, or like you're tipping back too much?
If your saddle is too narrow, the front will not be able to fit down on the back/shoulders of the horse, and the whole saddle will tilt back. You'll be able to tell, because it will be hard for you to ride correctly. You may also have this feeling of being unable to pull your legs back underneath you if the saddle seat is too big.

When you ride, do you feel like you're tipping forward? Does the pommel feel low? Your saddle may be too wide.

When you ride on a well balanced saddle (a proper fit for the horse), do you feel like the cantle is pushing you from behind?
The seat may be too small. If the seat is too small, you won't be able to move back far enough in the seat and you may feel the cantle is crowding you.

When you ride, are your knees poking out in front of the saddle flap, or do you feel like you can't put your stirrups up as much as you'd like? If you do put them up, are you pushed back against the cantle?
You either need a different style of saddle, with a more forward flap, or else you need a bigger seat that allows you to move your body back. You may have a long hip to knee measurement.