Nueces5:
I've never seen confirmed pewter crack like that item.
Scattered in that huge thread referenced above (at 20 posts per page, 74 pages is still a lot to look at. Fortunately it's my favorite kind of reading - lots of pictures!) are useful hints on the physical characteristics of unmarked pewter. A lot of the thread shows various touch marks that confirm that the pictured item actually is pewter (because, like, it's says so right on the bottom), useful for the unmarked items because you can get a sense from the pictures of the real thing what form a lot of pewter takes. So if you have an unmarked piece with both the right physical characteristics and a typical shape, it's a good bet (but not a sure thing) that it is pewter. If doesn't have the characteristics and isn't a common pewter form, the chances are much lower that you want it for your casting pot.