If you have a smartphone (aka: iPhone or Android), there are a couple of tachometer apps that all seem to cost around $5.
I use one for my heli and it should work for fixed wing too.
It is an audio tach and just needs to be close to the engine. I typically set the phone down on the ground and hover over it. They are adjustable for blade count.
I also use a shutter style optical tach. You point the view finder at the blades and then rotate a dial until the blades become motionless in the view finder. The RPM is displayed and holds until you press the reset button or it turns off. It basically takes two hands to do the measurement and requires a buddy standing next to you to do the measurement if you are flying a heli. But, with a fixed wing, you could anchor the plane to the ground and do the reading solo.