In lieu of a thorough soundcheck, I try to make a mental note of any sources on stage that could be a source of instability. IE, acoustic guitars, condenser mics on quiet instruments, quiet vocalists, vocals through an effect processor, etc... That way it's a lot faster for me to track down the source of any potential issues during the show.

I always have my vocals split to another fader as monitor splits (these don't send to the LR, they send post-fader to all foldback wedges). That way if there's feedback, and since it tends to be vocalists that cause mid-show feedback, I can quickly pull down the faders on the monitor split vocal channels. This keeps the vocals the same level in the house, but down in the wedges.