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.