Hand Held Tach

[Coasty]

http://www.mfgsupply.com/m/c/32-9073.html

I don't know if I has previously posted the above link is for a hand held tachometer.  I have been using this model for many years, great for bikes without a tach especially when setting up a SERT/SERF or other fuel management devices. 

[Panzer]

I have one also.
Was in the mower repair business, back when, works well.
Just get it near the plug & read.