I use an inexpensive diagnostic tach to check my idle speed now and again.
I've got an ENM digital tach, Model PT15B2. This one:
I've got the wire wrapped around the spark plug wire, with the free end tucked away.
When I need to set the idle speed, I just hook the tach up to the wire and set the idle speed. When I'm done, I disconnect if from the wire and put the tach back in my drawer.
Simple, cheap, unobtrusive.
IHTH someone!