I found some time to prepare a bench and take some pictures to well explain the method I'm using for more than 10 years now to set the neutral position of the control surfaces on my planes.
This is using the light reflection on the underside of the wing. I'm positioning a bright and straight object (a ruler for exemple) perpendicular to the wing on the table in a way the object reflects on the underside of the wing. You have to look in the axis of the ruler.
If you see the image "broken" with a shift at the hing line, even a little bit, then the neutral position is not good.
The setting is extremely accurate using this method. The precision is one trim click !
When you are at the slope/field and if you don't have a ruler with you, you can do the same using a long blade of grass.
Hope my explanation is clear enough and will help.