So we know of two meteors that hit earth from outside the solar system. (One was IDed by the US DOD as traveling well in excess of the solar escape velocity.) They’re what Avi Loeb is bootstrapping a search for now. If those can be recovered all sorts of things can be tested and identified. It’s possible interstellar meteors are already in inventories and even studied but we don’t know what to look for to ID their source as such, yet.

But more directly, absorption spectra are so intimately tied to the QM of the molecules and elements involved that they’re probably unique. (Hunch/not seen a proof.) This is also the most experimentally verified theory known.