I just used one of the handhelds on my diesel this weekend.
Not sure if this is right, but I put a little piece of the reflective tape (comes with the handheld) on the flywheel.
If I had the monitor too close, I got some funny high readings. I think that is because I was getting too much reflection from the non-tape metal area of the flywheel. So I backed off about to 6-8 inches off and got good readings.
The weekend before I used an oscilloscope and a piezo thingy. You flick the piezo thing with your finger and it makes a blip on the scope. I put a tape flap on the flywheel and let it hit the piezo thing every time it went around.
Also measured the drive shaft RPMs to to calculate my gear ratio.