Full Idea
One suggestion is that any two different substance, however alike, exhibit a positive entropy change on mixing. So absence of entropy change on isothermal mixing provides a criterion of sameness of kind.
Clarification
'isothermal' means at the same temperature
Gist of Idea
Maybe two kinds are the same if there is no change of entropy on isothermal mixing
Source
Robin F. Hendry (Chemistry [2008], 'Micro')
Book Reference
'Routledge Companion to Philosophy of Science', ed/tr. Psillos,S/Curd,M [Routledge 2010], p.522
A Reaction
[He cites Paul Needham 2000] This sounds nice, because at a more amateur level we can say that stuff is the same if mixing two samples of it produces no difference. I call it the Upanishads Test.
Related Idea
Idea 8153 By knowing one piece of clay or gold, you know all of clay or gold [Anon (Upan)]