Generally cracks in Mag heads are easy to spot after a good wire brushing. You can also spray starting fluid on them and watch the area between the seats. It evaporates quickly, but will stay in the crack a might longer.
I've pressure checked a few Mag heads that were cracked and no leaks, but like PHR said, I wouldn't try to sell any work on them.
Yeah, I usually pour rubbing alcohol into the ports and see if any gets past the seats over the course of an hour. I sometimes I shine a flashlight in the port in a dark room and see if I can get a sliver of light around the valves.
