Radial arm saws often get a bad reputation as being inherently inaccurate. But with proper set up and usage, this reputation is an undeserved slander.
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
Accurate work on a radial arm saw requires good contact between the fence and the work piece. Sweep away any dust and chips that might lodge between the work piece and the fence. Each time a cut is to be made, sweep away any dust or chips first.
Step 2: Make a couple of square pieces
I have some thin plywood left over from another project. Cut two squares about 12 x 12 inches. The machine cut edge of the plywood is against the saw fence.
Step 3: Mark the factory edge
Before you forget, mark the factory cut edges.
Step 4: Mark the square corners
The square corner cut in step 2 will be the standard of reference. Mark it on both pieces so you can identify it easily later.
Step 5: Stack the two pieces and cut
With the two pieces stacked over each other, trim the edge of the two pieces with one cut.
Step 6: Flip one of the squares over
The upper side of one of the squares will be flipped over so the edges cut in step 2 meet one another.