Mainly from the amount and complexity of symbols. That gives rise to the context which they must include. We can tell from the other more complete examples we have seen that writing systems which are complex enough to reliably convey such a wide range of context generally require being refined over millennia. It's not a 100% given, but it's a very reasonable assertion.
Numerous and complex symbols are characteristic for the earliest scripts.
The refined scripts typically use fewer and simpler symbols. The only exceptions to this tendency towards standardization and simplification are in the case of some script variants whose main purpose is to be decorative, not practical, e.g. which are intended for inscriptions on monuments.