So McKnight, sounds like your group is a bit like gits about character. I would have you face the wrong side of my viking axe if you claimed your puny dagger was infact a greatsword when I crashed into the character, and your model would be in grave danger, I tell you.
You tell if a model is armed with an obvious weapon or other item that will be there.
If he gots a helm and you make it be helm of the Skavenslayer and don´t tell me, yes fine, the dude had a helmet. But if he was bareheaded and did not even have freaking helm hanging on his back, you might be in trouble. Just telling yaa....
No, you would not need to tell me it was the helm of the skaven slayer, but you´d better explain he has a helmet somewhere.
If it is not obvious the fellow is packing a greatweapon or a bow, and is supposed to have those, you ANNOUNCE that to your opponent before the game or you are a git.
If he got a hand weapon of some kind and you gave him a magical sword or whatever, no problem, as long as it counts as a hand weapon in the weapon rules. Now, if you would say the dude suddenly packs the Dragonbow and ain´t even got an arrowhead on him, boy... Don´t play with me.
Gem, parcels, magical shoelaces, whatever, sure, no need to tell cause they could be hidden everywhere, but when it comes to big stuff. If your opponent don´t have visible armour, at least say he is supposed to have armour. If he has a greatweapon and it is not represented on the model, please be so kind and tell me.