Uh, in-built PIP-Boy 2000 encyclopedia? Scanner? Read the manufacturing stamp on the weapon?
There's a myriad of possibilities, all fitting in with the universe.
I disagree that it's fitting with the universe. Playing the game with an intelligence of 1 where your character's dialogue is little more than "wubba wubba" would lead me to believe he wouldn't even know how to read about something from the PIP-Boy 2000.
And how does he use a weapon? Or how does he make a difference between torso, groin and head? And so on, bla bla, yadda yadda.
Pleeaase, those descriptions are directed at the player, the character and the setting only acknowledge their existence and handwave it.