I once did a passable job by sitting on the floor in front of the bike with one foot on each pedal and pulling really hard. I took some careful measurements before and along the way.
I had a nearly identical bike to compare it too and it rode pretty much the same afterwards. Before, it rode like a real dog and I couldn't figure it out. It was a subtle bend. Maybe a couple of CMs back and 1 CM to the left. I think the left hand bend was the bigger issue as far as the "doggy" feel as it forces you to ride with the wheel dragging a little sideways to compensate.