if people stay in the back of the room, you're probably too loud.
if people are up front and walk away from the speakers with looks on their faces, you're probably too loud.
if people keep saying "WHAT?!?!?!" to each other, you're probably too loud.
in most smaller places, i match the PA to the loudest instrument (usually snare, cymbals, or a loud guitar).
in medium-sized places, watch the crowd's reaction.
in larger places, ... who cares?! :-P
just watch the crowd and see how they react. it's not rocket science, ya know?