Setting up a mixer, microphone and different types of stage monitors how we would for a vocal mic we tested this technique. Slowly increasing the gain of the input until we heard feedback we would then identify the frequency of the feedback. It is usually naturally obvious which range of frequency the feedback is in.
"Hoots and howls: Likely in the 250 to 500 Hz range.
Singing: The range is in-line with 1kHz.
Whistles and screeches: Most likely above 2 kHz."(2)
Once we roughly knew the feedback frequency we used the desk eq to cut it. The mixer we used has 4 bands of eq. (quasi parametric eq for the low mids and the high mids and fixed frequency eq for the lows & highs). By sweeping across the frequency range we found the feedback and cut it by an appropriate amount. We then increased the gain and until it began to feedback again and repeated the process.
How effective is controlling feedback this way?
We we're able to get a good level through the monitor and the input gain was at about '2 o'clock' before we couldn't turn it up anymore without feedback that we couldn't control.
What are the problems controlling feedback in this way?
The main problem we faced was that the desk only had 2 band of eq that allowed you to adjust the cut frequency. This meant we could only cut two frequencies using them. It also meant we would have no eq when creating a FOH mix.
Using a few different monitor speakers we noticed they all feedback at different frequencies and the mic took more or less gain to get feedback from them. This wasn't a problem because we were only using one or two monitors at a time. But during a performance where there could be a higher number of monitors it could be problematic.
How could it be made more effective?
1 - https://electronics.howstuffworks.com/gadgets/audio-music/question263.htm
2 - http://www.behindthemixer.com/how-remove-audio-feedback-through-equalization/
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