Monster Knowledge: Upon encountering a monster for the first time, have each player roll a die... the exact size is up to you but I'd say that it oughta be at least a d6 and probably no more than a d10. They know a number of important facts about that monster equal to the number they rolled. Start with the players who rolled lowest (come up with your own method of breaking ties), then keep going until you reach the person who rolled highest. Everyone who rolled the same or higher has a 4 in 6 chance of already knowing the same fact. Important facts equal things like one special attack or defense ability (troll regeneration, for instance), the number of them that tend to group together, basically any statistical thing that can be rationalized in terms of actual in-character knowledge. Creatures that have a reason to be obscure (for instance because they're extraplanar, very rare, or newly created) might grant a penalty to the roll.
If a character has a particular reason to know a lot about a given creature you might give him a bonus to the roll. If he's intimately familiar you might even let him ignore the "common knowledge" roll. This way your players are unlikely to go into an encounter completely ignorant of what they're facing. I mean surely they're not complete newbies, right? They've heard the campfire stories or read a few bestiaries or something. If you like you can have a chance of them knowing something false or trivial, but I wouldn't make it too high of a chance, the whole point is that PCs are reasonably informed about their own world.