So the obvious question to me is: is there no other physical property that can be used to separate the hydration shell + ion molecule combinations from the just-water molecules?

Different magnetic charge?

Different mass?

Different chemical reactivity?

¯\_(ツ)_/¯

I know the answer must be 'no' since if there were a better answer, none of what I'm thinking of requires more than high-school-level chemistry to discover/exploit.