Depends on the game.
Many games use nerf rockets.
fired out of a launcher, sometimes just hit the tank for a kill, maybe first hit is disabled, then a 2nd hit for a kill.
Theres room for dispute there, the people in the tank won't necessarily notice they have been hit.
Coming in to vogue is to attach a fishing net or two, land the nerf in the net for a kill and it's clear that it was done.
Some games use special colored paintballs, issued by the person running the game, in a limited supply, and must fired by certain people with guns mocked up to look like rocket launchers.