Player knowing difficulty of an encounter?

I know that a knowledge check can tell you about the powers and vulnerabilities of monsters and stuff. Is there a rule for someone to know or even get a hint that fighting a particular encounter is much more difficult they can handle? Do any DMs do this or have a house rule on this?

The players have known since the beginning of the game that there may be NPCs, monsters, traps and dungeons that might be way above their level but I wont stop them into doing them. As this world has been going since 96 so alot i do not have to make myself. I am just seeing if I can give players the opportunity to look at a monster that may not be hostile and they may be considering attacking and then check and be like, probably beyond your abilities.