You equip a gun and play a strike or story mission to test it out in PvE. You use it in crucible to test it in PvP. You make note of the damage it deals by reading the numbers that appear on screen when you shoot a target. Not difficult at all, unless you're mentally deficient. People are already doing that, they use a gun and decide if they like the feel of it. Nobody uses Doctrine in PvE because it sucks there, but in PvP it's good so it sees much more use. Why are you even against this? It will only benefit the game.