Visual inspection of the muzzle under direct light will reveal if there is any copper present. Careful attention to the “feel” transmitted through the cleaning rod and a tight patch will indicate tight spots, which indicate fouling deposits. Copper dissolving chemicals that yield a colored patch is a good indicator of interior barrel fouling. Of course, any time a patch is pushed through the bore and emerges darkened, there is fouling somewhere. In short, if the patch isn’t clean, neither is the bore.