Dennis obviously knows what he's talking about, follow his advice!
You know what RPM the engine is running at with a tachometer...
You don't NEED a tach, but they always help. It's easy to hear the engine speed up and slow down. A 400-500 RPM drop is basically just enough to hear it noticably slow down...
http://www2.towerhobbies.com/cgi-bin...?&I=LXPT31&P=7