There would be lots of ways to do it, the primary object being to measure the distance between a line drawn down the middle of the fork column and the center of the dropout. Actually, I just thought of a method while typing this which I believe would work.

1. Place the fork on a level surface, pointing the rake down.
2. Take a short level that has a groove for placing it on a pipe (common on 9" levels) and place it on the fork column, tape it if you like. If a threaded fork make sure the level is secure against the unthreaded portion.
3. Lift the fork column up until it is level.
4. Measure the distance from the bottom of the column where the level is to the table/surface (at the end OK if unthreaded).
5. Add 1/2 of the fork column O.D. where the level is, subtract distance from the dropout center to the table.

I would think this method would be accurate within 1 mm.

I may be old-school, but my mechanic's brain is still working!