The SAFE way of measuring head speed on a helicopter is to do it optically.

Get youself one of these:
[link]http://www.modelavionics.com/tach.asp[/link]

You look thru a window and match the headspeed and read the number on the face of the device. I don't even leave my chair at the flying field when my buddy asks me to measure the headspeed on his helis.

Here is a picture:



Rafael