Scub:

All you can measure is static thrust, and the weight of the plane has no effect beyond the rolling friction of the wheels. As the engine is running at speed, push the plane forward until the scale reading goes up, then gently stop pushing and note the reading just as your hand comes off the plane. Then pull the plane back against the thrust, and carefully let the engine slowly pull it back. Again noting the scale reading when you let go of the plane.

Ideally the two readings will be the same, but if there is a difference it's caused by rolling friction, the true thrust is the average of the two indications.

Bill.