When you are hit you will either feel it or hear it. If it hits a soft spot, you will feel it, when it hits a hard spot like a vest or a filled pouch, it makes noise and you will hear it and sometimes feel it too. So unless you are shooting from so far away that the BB has virtually no power left in it when it hit, say, the dump pouch on the belt of a player, the player will call the hit.
Seriously, the only place where people sometimes do not feel it is if you hit the pocket where they keep their kill rag with an underpowered gun... anyway, if they don't feel the hit, shoot again, they will figure that they are shoot at quickly enough and call themselves out.
The protection of the vest is great in CQB though, especially in games with no freezes or MED. A vest will absorb hits from point blank, so if you are in such a game and you shoot from very very close, aim for the vest.
Keep quiet. Sound travels faster than BBs.
Québec province's master age verification representative.