Not much is worse than going to a concert and listening to the band spending 5 minutes asking the crowd if the levels are alright. It can get quite annoying when the band lacks the self awareness that the momentum of a great live show dies when you do this. If you want to be very stealth about this, a simple way to get the job done is put your cell phone down on the stage (works really great on an iPhone), and have a trusted fan/roadie/soundguy text you whats wrong with the levels. Communicate it to your fellow bandmates and you are off to a much slicker and less annoying live show. Just remember, if you have one of those phones that ticks every ten seconds, keep it away from your amp!