"Ignore the YouTubers that say "listen" because that's hogwash and you should be wearing ear protection anyway. "

I agree with Michael's advice with this one exception. You can and should listen to the tool while it's cutting. You can pick up the sound of chatter (even with hearing protection on), which indicates the feedrate being too fast or the rpm too slow. Whining or screeching tells you just the opposite - rpm is too high or the feedrate too low.