MarauderX
Explorer
The way I run things is to let the players know that PCs know some of the obvious things that others would know, such as vampires feed on blood. However, I also remind them that some is fact, some is fiction in the world I run (homebrew) and not everything works the same way. If something like spells are different I tell the casters upfront about it, but if it is about some critter they have never encountered before then they should have no way of knowing what has been modified or not. I also let the players know that the XP of such encounters has been modified to match, and that quiets up the munchkins pretty quick.