Take a piece of tin foil and roll it into a ball smaller then the bore of the rifle. Drop that down the bore and then shine a flashlight to the side. The light will reflect off the tin foil and somewhat illuminate the bore for you. When you are done looking, just tip the rifle and the tin foil will roll right out...

If after you clean it up and your not sure of the damage to the barrel, have it checked, or load it very light. No telling how deep the rust has penetrated.