Ditto Charlie.

You can look for obvious cracks, usually between the valves, or radiating away from the valve seat (although I once had a Starion head crack in the exhaust port). Or you could buy a cheap crack dye test kit and a u/v light source, or the machine shop can pressure test (mandatory anyway). Get a crack warranty (guaranteed rebuildable without welding) on the used head and all you'll be out is time and gas if it is bad.

Not responsible for advice not taken...