ORIGINAL: STUKA BARRY
Just fasten a digital fish scale to the tail of your plane and to an anchor point, start engine advance to full throttle, read scale measurement, its that easy.
LOL, it don't get much simpler than that! If you wanna see a really trick, nice setup, pick up a copy of RC Sportflyer magazine and look at Mike Hoffmeister's engine reviews (you can also find them on Youtube under RC Sportflyer, I believe). He has a ball bearing slide, digital scale, laptop telemetry setup that gives really nice data: rpm, thrust, engine temp, etc.