360 heads on a LA 318 vs Magnum heads on a LA 318

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.