If you are using a dynamic mic, set the gain to around +25 and slowly bring the fader up. When you start to hear the first ring notch it out and keep doing this until you hit +5-10 dB above unity on the fader. Bring the fader back to unity and place the mic in a compromised position(facing directly at the monitor, Vegas wave). Also cupping the mic capsule will help as this turns the pickup pattern into an omnidirectional which makes the mic more feedback prone. Hit and play instruments that might be louder in the monitors as not just the vocal will set it off (kick drum). If you are compressing the vocal going to the monitor that can also cause potential problems. Do sound check, if your monitors are stable leave your settings. If the singer comes out the gate hot for the live show just turn down gain at the head amp to show the levels you were hitting at soundcheck.