They can tell a lot by what proof marks are on the barrel, also if you have a 12 inch barrel that says full choke on the side , that is usually a giveaway. Also they do know what barrel lengths makers have made, so if you have a 13 1/4 barrel that says remington on it, you are pooched. Also most short barrels have a bead that is significantly raised (on a pedastel), so if you just have a bead on it. Bottom line is that a cut barrel shorter than 18 inches turns the gun into a prohibited device and the penalty is the same as if you were running around with a full auto uzi with a bandoleer full of 30 rd mags. The other thing is that if the cop on the beat is at all uncertain, he will just seize the gun legal or not and let the higher up's or the court sort it out