And how would you do this without saying "They are lvl 2 and too strong for you right now" ?
And then after leveling up
"You suddenly feel strong enough to take on the Redbrands."
That's a general genuine question I have about DnD
How can your PC know how powerful they are compared to other beings, especially humans?
That's not a question about D&D; it's a question about exposition. Characters learn things about potential opponents by looking at them, gauging their appearance against their experiences, and applying a bit of instinct or common sense. As the dungeon master it's your responsibility to make sure the players get that information so they can make an educated (if not actually intelligent) decision about whether to enter combat (or do
anything in game, for that matter).
Personally, I had the NPC the PCs met in the trailside hideout express limited concern for their wellbeing when they announced they were going after the brigands, comparing the brigands' military training and superior armament to the monsters' disorganization, small stature, and poor weapons. He did this because while they had leveled up they were still only 2nd level. He did not tell them not to go, but he exhorted them to be careful and wished them luck, the implication being that they would need some of it.
But even in the absence of the NPC, a 1st level PC should walk up the road, see that bar full of brigands, and know /immediately/ that he's out of his league. The brigands have muscle tone that speaks to years of combat training. Their armor is expensive, and in good repair. Their eyes have that peculiar animal darkness that comes from watching men and women who deserved better die on their blades.
Can you ever be 100% certain that players will take the hint when you try to warn them off a deadly encounter? No. Even the players who aren't imbeciles are still at the table to take risks. Risks make good stories. But if you run an encounter that ends in a rout or a TPK, and the party complains that they didn't realize the enemy was that powerful,
you didn't do your job.
When comparing to video games:
In WOW you could hover over the name and see the lvl.
In Skyrim you wouldn't know until it's too late and you're getting beaten.
I like the last one better actually.
To each his own. The WoW system may be unnecessarily precise for feedback, but the Skyrim system is just crap game design, and it would be whether the game were tabletop or electronic. Real people have senses, and they use them. As the dungeon master or the programmer, you are the medium for those senses. Robbing players of those senses because you're not
required to let them be used just smacks of cruelty or laziness.