Perhaps an NRA card, and not bringing any guns to such a march, may be a start. If folks spend a few bucks to join the NRA, that should weed out the ones who aren't fo real.
There should be a way to fast track a person so they can join and have a receipt to join the protest. This way it keeps out anti's "I don't think they would join the opposition just to go to a march". Either that or have men "or leo's" do a quick check on everyone coming in, "like a gun show". Just preliminary thoughts, open to ideas. An accident would make everyone look stupid, we sure don't need or want that.
Probablly the only way to prevent it is not to have our guns.