I'm not sure there's a really accurate method other than looking at the specs for the fork.

The only other way I can think of is to use something long and straight (meter stick maybe?) and line it up perfectly with the steerer tube. Then take another ruler and measure how many mm it is from the "extension of the center of the steerer tube" to the front drop-out.

That's gonna be really hard to do accurately though. And the differences in forks are only about 5 mm, so it matters a lot.

Just look up the specs.