I don't necessarily discuss all house rules. I think there are a few that the characters would not know about, and that they can discover in-game.
However, the example you give probably isn't one of them. DR is not really "beyond character knowledge checks", as some skills specifically give characters knowledge of monster traits. There are some monsters that are pretty well known and understood in most fantasy worlds that have DR. Trolls, for example - eveyrone in a D&D world has heard of them, and their famous vulnerabilities and strengths. They are tough to kill, but not because their strengths are secret. Thus, the DC on these checks should probably be low enough that the general mechanic for DR will be known.