Be sure to have your horse on level ground when testing for saddle fit.
Once the horse is level check to see if the saddle is level on your horse's back. Start by placing the saddle (without a pad - but you can use a thin sheet to preserve the newness of the saddle) forward on your horse and have it slide back into it's resting place. At this point the saddle should be level and the front of the saddle should be just behind the horse's shoulder blades.
How is the skirt length? Go ahead and put the pad under the saddle. The pad should have 1 to 2 inches clearance on all sides. The skirt of the saddle should not rub the hip.
Is the saddle sitting too high or too low? You should be able to look down the length of the saddle under the pommel and see daylight (if you are doing this at night - turn on a light). You should be able to slide 2-3 stacked fingers under the pommel. If you can slide more than four fingers the saddle is too narrow. If you can only squeeze one finger than the saddle is too wide.
Is the saddle pinching the withers? You should be able to slide your hand between your horse's withers and the saddle skirt. [/*][/*]
How does your horse like the saddle? Does your horse try to nip at the saddle, or pinn its ears back, is there bucking or other behavior that indicates the saddle is not fitting? If so - the saddle likely does not fit.