With the two squares pushed firmly against the fence or any straightedge, slide them laterally into each other. Look for any spreading at the front or the rear that makes a slight "V" between the pieces. A knife edge allows evaluating how much of a "V" exists. If the "V" is at the fence, the saw arm is too far to the left. If the "V" is at the back (near the saw operator) the saw arm is a little too far to the right. The separation noted in the "V" is actually double the amount the arm is out of alignment with the fence. See also my previous Instructable
on a way to make small adjustments in the alignment of the arm with the fence.
The alignment on this saw is pretty good. There may be a "V" gap of a few thousandths of an inch near the operator, but it is so small over 12 inches that glue would easily fill the gap.
This method can also be used with a table saw, especially if a sled
is used in place of a miter gauge.