Another way to measure side thrust has just been published in Model Aviation and goes like this....Measure the distance from the tip of the prop - when the prop is horizontal - to a center point in the rear of the plane, such as the center of the fin at the base of the fin. The distances will be the same if there's zero side thrust. The numbers that are given to establish how much thrust there is are, if using a 12" prop there is one degree of side thrust for each 3/16" of difference in the measurements. I haven't checked the math.

Seems like a good way of checking to me.

By the way, while I realize that most folks do go through a pre-flight and that the ailerons are part of such an activity, I have also seen folks do the pre-flight from the front of the plane and tell me that everything is correct when they should have been at the rear of the plane and seen that the ailerons were reversed. It's a very common mistake that as an instructor I have had to correct on many occasions.