I was taught to place a mic on a stand near a monitor speaker, then slowly increase volume on that speaker until feedback starts, then start fiddling with the EQ faders until you find the fader that will eliminate the feedback. Slide that back a notch until the feedback stops. Then increase volume again until feedback starts again, rinse and repeat until you can really turn the volume really up on that speaker until even a mic cupping singer with hearing problems will be satisfied. Chances are you will experience feedback all over the spectrum, but most of it in the middle. A good sound engineer can do this fairly quickly. And even tell which frequencies are causing feedback from hearing alone. The only frequency I recognize is 1K, as that used to be the one you'd hear on TV when there was nothing on, except that test image. So at least I could eliminate half of the frequencies when looking for the culprit.
As for the app, don't use it. You got to learn how to identify feedback frequencies yourself before using (and making yourself reliant on) an app. Practice, practice, practice!