I just use the low sound vibration just before blade stall, then I know it's a little to low and when it's to fast you hear a high whiny sound, just the same sound a pilot makes when he has to do ground runs that day
And if it's a robbie be carefull that no one is cutting the grass near by, as you might get confused with the lawn mower engine and think you are having an engine failure....
Robbie's got to love'em
spent a good few years in them.
fluffy