How to balance the need to keep a not-always-level-appropriate environment fair to players, with the need to keep a sense of mystery about the campaign? There has to be a middle ground between the monster-book memorizers and the clueless neophytes who unwisely take on a troll.
Most of the people I've been DMing for recently are new to the hobby, and I've been trying a variety of subtle and not so subtle hints. I think one thing that might work is just a simple rule of thumb based on size, with cautions about oddball monsters and special powers. This says nothing about what kind of bestiary the DM is working with, but avoids some of the more obvious mismatches, as long as you refrain from throwing killer bunnies into the mix. By the way, fighters should also be able to size up the fighting capacities of other humans and beings with levels.
I ran out of room to mention dragons on this sheet, but there's no real need to, right?