Definitely assess.

Apart from the fact that girls often hide their abilities anyway, you also have the second child factor involved... later children often try to find a niche for themselves that isn't already occupied, so if the oldest already had "the smart kid" locked up, she may have decided not to bother.

Plus, these things run in families, so you might as well check anyway, because there's already a high probability she'd gifted just based on genes.