Pressure testing is used to check the integrity of the water jackets.
My suggestion is to have the heads disassembled, cleaned, magged, and given an overall evaluation.
However, if you want to do your own pre-inspection, pull them apart and with some carb cleaner and a small wire brush that goes in a drill, clean off all around the exhaust seats in the bowls and chambers.
Then carefully look for cracks in the seat area with a magnifying glass. Those 302 heads are rather crack prone, and if you see cracks, stop right there and move along to plan B.