If you teach him to identify weapons (as part of speak) then he'll be able to tell you that people have them, and if not too many, count them for you. Of course he'll only be able to identify things as weapons if you've trained him to recognize them. If you find all sorts of different weapons to show him and train him that "these are weapons" then you could do that with one word. If you want him to know gun, sword, bow, spear, etc. then each would take it's own word.

Potential issues might be if you lump too many things together as a weapon he might think some other common tools are weapons, but if you teach him many different groups so he excludes common tools then if there is a weapon he hasn't seen he won't recognize it as a weapon. Both options should be pretty good, but they're not going to be perfect.