Great question! I swear, I eyeball it, tightened it, eyeball it again. Go for a short test ride and it's slightly out to one side or the other. That's when I really find the truth in alignment. I loosen it slightly, and just tap it in the direction I "perceived" it to be out. Then test ride again. Seems at that point my body or alignment of my shoulders and arms in relationship to the alignment of the bars and front wheel tells me whether it is right or wrong. That usually does the trick. Test ride will always tell.

If there is a trick other than eye balling I'm interested in applying it to my bike mechanics too!