Originally Posted by howardcano

You can get a photo-tachometer for about $10 on the evil auction site:
http://www.ebay.com/itm/Digital-Tach...Whc&rmvSB=true

To use with a brushless motor, get a green or yellow LED, wire it in series with a current limit resistor (470 ohms to 1K ohms is fine), and connect this to any two stator terminals on the motor. Face the LED into the tach, and off you go!

Here's one that I made a while back. I put the LED and resistor inside, held against the tach's photoresistor with some shrink tubing and electrical tape, and brought out the wires to a pair of alligator clips for ease of use:

]