Lash the tiller cross bar to something so that the rudders appear to be straight ahead. Then with a pencil draw a line
parallel to the waterline down X inches from the bottom of the hull. (this presumes that the rudders are locked in the down position, right <img src="http://www.catsailor.com/forums/images/graemlins/laugh.gif" alt="" /> )
Now measure the distance between the center of the leading edges at the line, and also the trailing edges.
If the rudders have no toe in than the distance should be the same. 1/8th inch of toe in would mean that the leading edge is less then the trailing edge by 1/8th inch.
You can check your Ackerman steering in pretty much the same way just move your tiller over half way and measure again.
For the basics, go to
http://www.rctek.com/handling/ackerman_steering_principle.html