> What is the most reliable letter to drink a potion?
'q' in most roguelikes, but I think there are some with something like 'd', or 'u' (use) or object-verb.
> What do you see here?: "@"
If you mean the player character, that's also not in all roguelikes (I mean graphical ones). I am not sure how common is the knowledge that @ is used for the PC among graphical RL players.
> What's short for Deep Dwarf Necromancer ?
Such abbreviations are used only in Crawl (and maybe some other few games).
> You could have a question for each of the majors and select one randomly. That way if someone tries to register and a weird one pops up, they can just refresh.
What if somebody plays only minor roguelikes for some reason? Could be risky.
Maybe something like: "Many roguelikes have a feature that you need to start a new character after death. How is this feature called in RogueBasin?" or "Where was the first International Roguelike Development Conference held, according to RogueBasin?"