Mount motor in plane with prop attached.

Turn prop until blades horizontal

Measure from blade tip to wing leading edge on both sides. Should be equal .

Some tips and explanations.

Use 2 setsquares, one at prop the other at l/e and measure between the verticals.

If the plans require sidethrust (not common on rc models, but...) calculate the difference between the two measurements at prop tip (it will probably be a matter of millimeters...

Make sure that you have some means of getting the wing into exactly the same position easily every time you assemble, including at the field. It only requires 4 small balsa (1/16 sq is good) "keys" glued to the wing so that they sit snug against the fuse when everything is in the right place... Same with the stab if it is not glued on...