Isn't all this solved with monster knowledge checks? The DC to get level is pretty low; if you hit that DC but still don't get it the monster must be shrouded in more myth and legend than fact, meaning it's probably out of your reach.
It's an in-character check, too, because you're saying, "What does my PC know about this?"
And you just beat me to it
Honestly as a DM who has run mostly sandbox type games in a setting that is now 30+ years old (so there is really a vast amount of "stuff" of all levels here and there) I pretty much follow along with what ExploderWizard is saying. Lets look at it this way.
Suppose the party gets wind of the existence of a white dragon living in a mountain pass some distance from a town. Maybe they see it flying in the distance or meet some people that it attacked, etc. Now, in the real world what would rational people do in a situation of that sort? (well obviously IRL no dragons, but we can still think about it). They'd probably first try to find out as much as they can about it. First line maybe is just plain monster knowledge checks (some dragons are really bad assed and we don't want to mess with them). Next is going to be probably say a history check (I don't recall reading about any dragon living in this pass sayeth the warlock). Next is going to be some actual research (skill challenge, go find out who DOES know about the area, convince them to answer questions, successfully draw the proper conclusions). FINALLY the last thing the PCs are going to do is go up to the mountain pass cautiously and stealthily (maybe even sending a couple PCs ahead on a scouting mission) to find out exactly what they're up against and if its defeatable to fight it.
Now, when it comes to the point where PCs are actually observing monsters and trying to figure if they're too dangerous to mess with, well there are a whole slew of things you can do here. First of all again is monster knowledge (look at that dragon, it must be 500+ years old by the color of its scales!). Second is just description, what size it is, what is it doing, how does it react to a threat? Describe the bones of monsters the party has already found to be dire threats scattered about the beast's lair (hey, this thing eats hill giants for lunch). Show the beast in action (wow, look at that whole tribe of hobgoblins run for their lives).
Finally as EW said, in 4e its pretty easy for a party to mostly escape from a seriously hard threat if they aren't really stupid. Yes, a character or two may perish, but there is always the option to take prisoners, etc on top of at least the rest of the party can usually scatter and escape.
In other words the only issue really with overpowering threats is when they're presented in a situation of win or die and then its pretty obvious that's not the way to DM if you want a good game. Other than that, the players should be smart enough to work things out for themselves and you as DM have practically always got AMPLE opportunities to give them the hints they need.
This applies with NPCs too. What fool is a player who's character simply approaches an NPC of unknown identity and thinks its a smart move to mess with them? At least wait until the party know-it-all makes a skill check (better not mess with him Bob, that's Ragnar the Red. He killed a cloud giant single-handed). Going off half-cocked isn't a survival trait in the real world, nor in (at least my) game world. Again though, should the party be so ignorant or unwise as to tangle with the wrong people, then at least 4e gives them a pretty good chance of surviving the experience.
I don't really feel bad about killing off characters that act like idiots I guess is the bottom line and in a really dangerous fantasy world there is a premium on caution.