-Flat surface. Granite countertop would be good enough. Concrete floor probably not.
-Flat object at least ~5mm thick and a minimum of about 100mm long. The longer the better, and the flatter the better.
You'll want to lay the fork down so that the steerer is resting on your flat thing, which is sitting on your flat surface (you need to do this to keep the crown race out of the picture). Get the dropouts sitting the same distance off the surface, then measure to the center of the dropouts, and the center of the steerer. Subtract. This is your rake number. If you do a careful job, you should be able to get within 1mm or so pretty easily.
There are other ways to do it as well, but I'd say that's the easiest without having any fancy tools.
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