We see phases of Jupiter, same as we do for the Moon, Venus and Mercury, but because it's further out than us, we see a limited range of them. This shows it to be spherical
Humankind has sent planetary probes to and past Jupiter and its Moons. These show various phases that we cannot see from Earth, and again shows it to be spherical
Hundreds, perhaps thousands, perhaps millions, perhaps hundreds of millions of astronomers have watched it and catalogued aspects of its appearance, behaviour, etc. None appear to have reached the conclusion it is anything other than spherical.
At some point you have to accumulate the balance of all the various evidence bases, and conclude that it all adds up to a spherical Jupiter, surely?