I would stop firing cheap ammo if I was worried about it. I have had problems with Sellier&Bellot cracking in other calibers. The brass was too hard around the neck. Sometimes had problems with Lapua also. Old Japanese brass made by the Chinese in the early 50s always cracks. If it is a minor crack it will do nothing. A good sized gap will eventually burn pits into the chamber and give extraction problems, but it will take a long time. As far as checking the headspace, that is probably not going to tell you anything because that round is called off the rim. I would buy better ammo if you are worried about it.