These devices are very simple.

Many of the popular RC magazines show them in use.

One easy way to do this is a simple fish scale anchored at one end, and the other tied to a plane.

A bit more elaborate is a normal weight scale, with an "arm" ( like an upside down "T" ) on the scale, and a hinge at the other end of the "T".

The motor is attached via a line, etc. to the long part of the "T".

As the prop "pulls" it places a downward force on the scale, indicating thrust.