Silly question. Use the secret handshake!
Oh... and I forgot to add: If the shooter is female, don’t worry - Laz will hit it and she will have to stop shooting!
Just a thought: The guy or gal shooting willy nilly, especially targeting innocents is probably the bad guy. The guy moving in a tactical manner attempting to close on the willy nilly guy in order to observe or engage is probably a good guy.
I also teach rapid response. We teach to respond to the gun shots.
That being said: keep your firearm either concealed or in low ready. I would follow all of the advice previously listed and make sure your loved ones are secured or evacuated.