I find the distinction between in-character knowledge and out-of-character knowledge often misleading.
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I think a vague narrative description of an encounter level that is only a bit above their level will not differ much from one that is barely likely to lead to a TPK. That is, unless you use certain keywords or some kind of short hand so that the players can decipher the narrative into more specific information.
I agree that the incharacter/out-of-character distinction is not all that helpful, especially in a game that is as metagame heavy and metagame driven as 4e.
In that case, same question with respect to encounter level: how do players distinquish between encounter level +3 and encounter level +5, for example?
On this, I expect the players to (i) trust me, and (ii) suck it up! They're playing 18th level paragon PCs who have an absurd depth of resources and capabilities to draw upon, and I expect them to deploy those resources. I also expect the players to draw inferences from what is going on in the fight: for example, if it is round 3, the hydra still has a couple of heads left, and the salamanders have taken close to 100 hits each and are still standing, plus more salamanders are approaching, I expect the players to infer that they need to pull out all the stops.
At 1st level I would be more forgiving, but would achieve this at the encounter design level - ie no 6th level encounters for 1st level PCs - than at the "deliberately feed info to players to facilitate their choices" level.
You XPed my hydra encounter post (thanks!) which shows my method at work. Here are
two other, earlier posts that illustrate the same approach, of pouring on the pressure and expecting the players to work it out.
If I was going to try and flip this around to answer the question in your OP, which is framed from the player's point of view, I would say: know your GM! If your GM is running in "lair assault" mode, where everything is to the death, and TPKs are the players' problem, you might have to invest heavily in monster knowledge, use that to work out the attack and to hit of the monster powers, from that work out their levels, and then from that work out the encounter level.
If you've got a GM taking an approach more like mine, you can be more confident that, while a defeat will be a defeat, it won't just be a "TPK brings everything to a dead-end" defeat. I'll always find some way to keep things moving, even through a TPK.
But I'd still say, if in doubt, use your resources! Cause you can't use them if you're dead!