Funny, I had the same dilemma. Solved in as follows:

I put the plane on the floor, and attach a wire to the tail. I made a see-saw contraption with two balsa sticks at 90 degrees on a pivot point in such a way that the more the aircraft pulls, the more a balsa stick pushes down on an electronic scale. Voila, thrust in ounces, pounds, kilograms, whatever you like....

Dutch engineering, I tell ya.

Hans