Immolate said:
They won't know, at least at low level. At higher levels, gear will become a tell.
At higher level, gear will only tell you that these will be uber-heroes, and that you are chewing too much than you could ever bite. Especially if they have weird abilities with which they can cheat death for a short time, or fly around, which wizards might very well have if they're paragon level.
Still, what is the advantage to fighting two-on-one on a melee character when there are squishies who are unmolested and raining death on your allies?
The advantage of ganging up two on one is that you're going to finish him off twice as fast... normally. That tactic works in the real world too, after all. It just happens that in the action-driven D&D-game, that little halfling dude isn't your run-on-the-mill NPC halfling loser, he's a player-controlled halfling hero with player character classes, and both Gark and Nark are in for a surprise. Causality works against them, however, as they are controlled by the ominous being, the gamemaster, to provide an entertaining challenge to the friends of the gamemaster, who all happen to control the little band of merry heroes who Gark and Nark are attacking with their orc buddies.

Gark and Nark will wonder why the little Halfling knows Sword-Fu. Or perhaps they'll die before thinking about that.
Most semi-intelligent melee monsters are going to engage the defender with one brute and send the others after the soft targets.
If they can pass him, and if they recognize that he's really tougher than they're bargaining for. I mean, the image we're talking about is of two brutish orcs, really tall, with muscles, who are afraid of a midget with a sword that might barely qualify as a dagger?
A halfling warrior with another defender could work as a team and shift together to give the halfling his advantage, but that's not always possible for any number of reasons. I think that the bottom line is that we will have to see. If halfling warriors really are superior overall, that reality will percolate up until everyone knows it. It'll take time though.
Heroic Halfling Warriors played by Billy are superior to Non-heroic Halfling Warriors from NPC-town, who live a modest NPC-life, until the gamemaster has use for them, where they either get mauled by horrible monsters for plot-reasons, or are NPC-hirelings... Or perhaps even NPC-enemies to the player characters.
Don't forget, the Feat Excerpt says that not every member of a particular race has the abilities that the higher-level feats provide, nor do most members of said particular race even reach paragon level. Some aren't trained in these techniques and art. Being lost in a crowd for combat purposes might very well need expertise for that. And being good enough to qualify as a Player Character Fighter is another thing.
It's like in D&D 3.X. More than 99% of all halfling combatents were level 1 Halfling warriors, at best. A very very few happen to have a superior class, like Fighter, Barbarian, Swordsage or whatever you want. But for most orcish raiders, nobody is really going to assume that Halfling Warriors are Fighters or so. They're either commoners with 1d4 hitpoints, or warriors with 1d8 hitpoints. And they stink. That's all. No problem for Gark and Nark. The same when the world is D&D 4th edition. Most halfling warriors die like minions, simply because that is so.
And then there is Hodo Bigguns... The last Halfling they're trying to rob... It won't end well for Gark, Nark and friends.
