Originally Posted by
Grumpy McTrumpy
construct a jig that mounts the fork upright on the headtube (flat board with an upright dowel, checked for square with carpenter's square). drop plumb line from the dropouts. measure the point where the plumb-bob intersects the jig base.
That's one way, here's another. It assumes the fork is out of the frame.
Lay the fork down on a piece of posterboard or paper, with the fork blades perpendicular to the surface. Mark the paper with the center of the dropout closest to the paper. Then find the center of the fork blade that's laying on the paper. Find at least two points as accurately as you can. A caliper really helps. Mark the fork plade at the center points. Take a level or straightedge, place the edge across the two points, and draw a line on the paper, as long as possible. Remove the fork. Draw out the line where the fork was. You should have one long line with a mark above it. Measure the distance between the mark and the line to measure the rake.
I should have made a video.