You need to determine an imaginary line from the center of your axle slots which runs parallel to your steerer tube (I would do this by laying the fork down on something and drawing an actual line). Then - without moving the fork - establish another imaginary line in the center of the steerer and measure the distance between the two. You'll may be off by a mm or two but you'll get close enough.



Another (maybe easier) way of figuring this out without removing the fork from your bike is to hold a string tightly from the center of your head tube and pull it as straight as you can to just next to your axle - basically following the line in the above diagram - and measure the distance from the string to the center of the axle.