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I would say in some cases you can tell, but more likely than not I think the assumption is that it broke after it fossilized. If you find the tooth still embedded in the original deposit and it’s broken, then it likely broke before fossilization. Sometimes there is evidence of bite marks on a tooth or feeding damage, which suggests the break happened before fossilization. If you find the tooth on a beach or in a river, it’s more difficult to be certain. If the tooth is really worn down, this likely happened after fossilization due to being transported by water (either on a beach or in a river).

Examining under magnification the surface of failed mechanical parts gives a pretty good idea of when and how the break occurred (brittle, ductile, fatigue, etc). This has been developed by looking at parts where you know how they failed. I would expect the same could be done with fossil shark teeth. Sacrifice a fossil tooth and a modern shark tooth by breaking them and use for comparison with future finds.