Next attack, I hit but some DR property of the armour saves him. "Clang".
Next attack I get a couple of points through DR, but many of a fighter's hit points reflect their skill in avoiding blows. So...."Clang?"
How easy should it be for a player to distinguish between these scenarios? Our DM is lenient is revealing that a firey sword has done no damage, but should the CHARACTERS be able to deduce fire immunity from that, given that its only(?) the die roll of the PLAYER that distinguishes two types of "Clang"? Is it likely that someone can meaningfully distinguish between the ways they are failing to affect their enemy? Is their any in-game distinction between DR and high AC, short of pathological trial-by-ordeal with a thousand pinpricks?