Don't bother, it's just a waste of time. The rudder counter balance is high up the fin, so you are suspending almost the entire weight of the model below your pivot line. An ounce or so of lateral imbalance is not going to show up against the many pounds of weight of the whole model refusing to be rotated upwards! Unless you can arrange the suspension points to be on the exact longitudinal line of centre of mass, forget it.

Flight behaviour is what matters, so trust what the flight behaviour tells you. If it gives all the indications of lateral imbalance, then deal with that. If it gives all the indications of a warp, then deal with that.

Perhaps there is no warp or imbalance at all. What I often find when asked to check out other people's models is that they only ever use aileron trim to correct a turning tendency which is actually a rudder problem. Consider a model that is yawing left, even with no roll coupling it will gently alter heading to the left. 99.9% of model fliers counter this with right aileron, so the model ends up imperceptibly yawed left, imperceptibly banked right, and seeming to go in a straight line but is actually unbalanced in a knife edge but with 1 degree of bank instead of 90 degrees bank. When you roll inverted the left rudder becomes right rudder from the ground viewpoint so the model yaws right, but the right aileron trim is still right roll and hence you get a good roll and yaw to the right.

It is essential to get the model in proper level balanced flight before making any other tests. I find the best way is to fly directly away from me and directly toward me, in line with the wind, and watch very closely to see if the model is yawing off the original heading and ensure that the wings are truly level. If it yaws off heading then trim the rudder, not the aileron. That probably will start to induce a slight roll and that can be trimmed out with aileron.

Trimming is only correct at one combination of speed and throttle setting. Change speed or change throttle setting and the trim is no longer correct, so do all your trim tests at the speed and throttle at which you normally fly the model. In the full size we are working the rudder all the time in aerobatics to keep the plane from yawing off line, in a plane where the prop rotates the same way as a model's prop it needs left rudder as speed goes above the trimmed speed and right rudder as it falls below trimmed speed, but we have the luxury of the slip ball to guide us.

H