You can measure by proxy the toxicity of a venom and it's measured as an LD50 value. Less venom required to kill something = more venomous animal.

But that value isn't derived from tests on humans, for obvious reasons. It's only a gist. And while scientists are sure that the Box Jellyfish is *really* venomous, the LD50 value of the venom from an Inland Taipan (an extremely venomous snake from Australia's Outback) actually suggests it's even more toxic than the Box Jellyfish. Calling the Box Jellyfish "the world's most venomous animal" is just bullshit sensationalism, since apparently "really really extremely dangerous" isn't quite badass enough.