For most games, most of the time, I use the system that popped up in the late 3.5E books.
Each 5 or so on a check gets you the most basic knowledge of the monster, starting with the most commonly known things and going up from there.
But it is always, tavern gossip, tall tales, stories, legends, and such. I make this very, very, very, clear to all players. Anything you learn from this roll is not 100% true, ever.
And if a player EVER even for one second tries to whine or cry and say "But..but you said when I rolled my monster knowledge check that the Zoomet was vulnerable to fire! Waaaaaa!", that character will have rocks fall on them and the player will be banned from my game.
I'm also super harsh with these numbers. Things that are common, by default anything in the core monster manual or in game that live around the area have low DC numbers. For something like a Zoar, I set the 'common knowledge' at DC 50.
I do much more encourage players to find monster knowledge both in-game and by downtime.