An Inductive Tachometer is a handy little tool to have in the toolbox if you are working on something which does not have a tach. Knowing the engine RPMs helps with setting the idle engine speed and tuning. Some folks are really good at doing this by ear, I am not one of those folks.
There are lot of Inductive Tachometers available – in a wide variety of price ranges. The one I have was really inexpensive and is very easy to use. Have never checked to see exactly how accurate it is, but I believe it is close enough for what I use it for.
This Inductive Tachometer works fine on the Sportsters and the the Honda Shadow. Just wrap the RPM wire around a spark plug wire 6 or 7 times.
The coil around the spark plug wire detects the spark when it fires to measure the RPMs.
Make adjustments to set the idle RPM as needed. The Honda Shadow should be set to ~1000 RPM at idle. One thing to note is the sampling rate on this Inductive Tachometer, it lags just a bit so make small adjustments and then wait between each adjustment to get a good reading.
The Inductive Tachometers could be mounted to be left on the bike or whatever, but I use it as a tool across multiple bikes.