Just point the plane to the sun and take the reading from behind the prop.

In low light conditions it can help to have a light coloured background to aim the tach at,with the prop rotating between the light coloured surface and the tach obviously...

In the dark, use a battery powered light to take readings against. Any AC powered light source will mess things up.
If you have a white or light coloured prop, you can also hold the flashlight behind the prop. It will reflect on the prop and activate the photocell.

What you are looking for with these photocell tachs is contrast...