One can use a bit of geometry to measure rake.
Place an axle in the fork, tie a loop in a piece of string and and run it to the top of the steerer. Measure the distance to the steerer's base, along with the steerer length. You now have a right triangle established. Now measure the entire length of the fork top to axle, and apply the same ratio of length to rise, and you have the rake.
BTW, don't forget to correct for the radius of the steerer.
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