Don't bother trying to fine-tune. Use this time to make sure all is as it should be. Experienced bands usually play a polka, schottische or other energetic couple dance to help with this before the caller begins, so you have fewer variables.Sanity checks
Check that all speakers are in fact working. You'd be surprised how often they're not. I've seen speakers blown because one was not plugged in and someone tried to jack up the volume to compensate.
Next check that all mics are working. At a minimum, tap each mic (don't blow into them - it's ugly sounding and not the best thing you can do for the mic).
Continually remind yourself to check the balance of the band and caller. If you're primarily a musician, remind yourself to check regularly how distinct the caller is in the back of the room, especially if they do a lot of squares. Could you dance to it? If you're primarily a dancer, make sure you don't have the caller booming out and the band meek - don't knock the stuffing out of the music.
Are all of the instruments audible at the back of the hall? If not, a mic or cable may not be working, or turned up.
Hums or buzzes? Look around for dimmer switches on lights, and either turn them off or all the way up. Flourescent lights on the same circuit as the sound gear can also cause problems. Make sure you're not using an unshielded cable where you need a shielded one. Turn channels down one at a time, briefly, to find a bad cable or other problem.
Look before Twiddling
Most feedback is a function of the shape of the hall and stage, and the placement of the people on stage and the speakers and monitors, rather than a mic aiming at a speaker. It usually is a problem at low frequencies, since objects are opaque to highs (where most of the sound information is) but allow bass to pass by.
'Ringing out the EQ' is an old rock band trick to set the monitors so that the loudest possible sound levels on a given stage can be achieved, by having the mics all on while the engineer plays with each slider on the graphic EQ while adjusting volume to the point where it begins to feed back.
You may run into sound people who think this is also a good idea for the hall mix with acoustic music - it's not. You want to get the sound as good as possible, not just as loud as possible, and this technique responds only to the resonant frequencies of the hall, not to what we want to hear from the music. But sometimes people resort to it to deal with...Nightmare Hall
Some halls are just too reverberent. The best thing to do is focus on overall volume, and err on the side of being too quiet, especially with the bass and piano, which generate lots of low tones which are very non-directional. If you have a graphic EQ, cut about 3 dB at 250Hz in boomy halls. Other alternatives: (1) use more speakers at lower levels, or (2) find a new hall!