I can't give credit since I've forgotten the source, but their idea was to take a steel straightedge about a foot long, and rest it on the edge of the top of the bar clamp. Then sighting along the edge of the tire to gage alignment. I didn't have great luck with this method, but it spurred another way to use a straightedge to align.

I hold the straightedge on the back of the fork crown and sight down the back edge of the handlebars.