I'm strictly an eyeball guy, but for those wanting more precision than the job needs, here's an easy accurate way.
You'll need a yardstick or straightedge 3-4' long. Lay straight edge across one side of the front wheel on a secant, and bring one end to the HB, noting the position with respect to the stem. Repeat on the other side, and compare, they should be equal. Note that it's necessary for the secant to be the same or close on both sides, so if the stick is long enough, use the fork tip as a frame of reference.
You might try an experiment, eyeball it first, then compare with the yardstick method. Odds are you eyeballed it pretty darn accurately, but at least now you know.
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