As I am a retired insturment tech and I can help you. You need a shaft encoder...which can be a wrap or a disc. Possibly you could drill a center hole in the end of the shaft driving the chuck or mount a wrap around a part of the rotating shaft and feed to an electronic measuring device.
Tachometers - AP1000 - Electro-Sensors, Inc.
I have no idea of price but I know it will work for you. As the drill becomes loaded and if speed drops...it will be noted on a digital readout and be very accurate! Electro sensors have every size and every application.