DISQUS

Leibniz has your back:

"Leibniz' Law", the formulation often referred to as "the indiscernibility of identicals" says:

if a = b, then for all F, Fa iff Fb

The contrapositive of this (if this thing has a property that thing doesn't have, then they're not the same thing) is your suggested test.

There's a correlative, more controversial, thesis known as "the identity of indiscernibles" that says:

if for all F, (Fa iff Fb), then a = b

The applicability of all this crystalline logic to the goopy semantic web, though, is a little more tangled ...