The photo shows the usual way of aligning and checking a radial arm saw for a square crosscut using a framing square. It is a good preliminary check.
Step 1: Sweep Away Chips
Step 2: Make a Couple of Square Pieces
Step 3: Mark the Factory Edge
Step 4: Mark the Square Corners
Step 5: Stack the Two Pieces and Cut
Step 6: Flip One of the Squares Over
Step 7: Push Squares Together and Check for a "V"
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.