You can't tell with a pad. :) Take the pad off and lay the saddle directly onto his back. Slide your hand down the front of the saddle, if it slides easily without much stickiness, that's good. Feel if it pinches his shoulders or withers. Put a hand on the pommel, and one on the cantle. See if it can easily be rocked from front to back. If so, not good. If not much movement, that's good. Lift the flap up, and see if you can see air under the saddle right above his back. If so, that's called bridging. That's bad. :) That puts two uncomfortable pressure points one in the front and one in the back due to the saddle not laying level on the back. Stand back and see if the deepest part of the saddle sits level. If the cantle sits lower, not a good fit. You can correct with a lift so long as it doesnt put unwanted pressure in the front.
If the pommel sits lower, it's probably too wide.
Hope that helps!