What objective evidence do we have that Musk is smart?

Basically: he's extremely rich

Working hard and being very good at negotiating compensation packages and picking the right companies/products to be involved with (and a bit of luck) are all sufficient for exorbitant wealth though.

E.g. there are plenty of people just as smart as Bezos who didn't hitch their wagon to the "sell something easy on the web" idea at the right time