Fallout – Who does Bottle & Cappy cheer up

There's the obvious of them having the same last name, but besides that if they're both in the bedroom and they're related they say something like"there's nothing like hanging out with family", instead of the usual pick up lines.

These are the only ways of knowing.

Outdated, now you can see that quickly when assigning a weapon to a Dweller. See other answer.

_{I still like my redhead squad, though.}

You cannot.

The only gun carriers you can quickly identify are those guarding the gate, since they have their weapons drawn.

The game gives you no quick way to identify all weapon carriers, so you will have to be organized.

You could, for example :

• Give weapons only when needed, then take them back (as suggested in on of the comments)

• Have your "defense squad" stationed in one particular area of your vault

• Give the weapon only to certain people : people with military suits (so RP !), women with red hair…