In 4e monster knowledge checks are also an easy way to drive the message home. Either they roll exceedingly well, so you can just tell them that they believe their opposition may be too tough to take on (yet), or you can tell them they have no idea what the heck they're facing...
I've come to the opinion that for 4e a successful monster knowledge check should typically give the PCs a ballpark idea of monster power; which Tier a typical monster of that type is, and whether it's Low, Mid or High within that Tier. Eg Low Heroic 1-4, Heroic 4-7, High Heroic 7-10. I think that is not too metagamey, and is better than "You can/can't beat this monster" which should always be up to the players to judge, and varies a lot by group - eg some groups will lose to an L+5 Solo, some will find it a tough fight, some will squelch it quite easily.
Eg the typical Orc is 3rd-4th level, so Low Heroic. The typical Gnoll is 5th-6th level, so Heroic. The typical Ogre is 6th-8th level, so Heroic to High Heroic.