[mrguyorama]

>IEMs

Absolutely. In orchestral band back in my high school days, between songs I would ask my friend on the tuba to give me a note for reference so I could tune the timpani. If you are in a band you should be able to manage to take a one second note and hold that by humming or something and tune up. I can't even read real music (I was a "percussionist" ie incompetent except at rhythms) and even I could do that. Better trained musicians than I could expand from there to the other notes you need.

You just need a single reference pitch. You can literally just play a sine wave into your headphones for a second on stage, or have your audio engineering guy feed you one. Or you have one of those dirt cheap tuners that clips to the fretboard to get you started.

The lead singer is engaging in stage banter partially to give you the time and space to do this. If your ensemble includes a pianist or a synth, you just have them slap a note for a reference.