How many years until we verify? 100? 1,000? 10,000?

Sounds like something we could check empirically. Some cosmic rays are protons from outside of the solar system. We could capture those and compare them with local protons to see if they're actually the same kind of particle. (We can check if two particles are the same kind or not by seeing what happens if we try to put them in the same place. If they're both protons, they should refuse to be put into the same state due to the Pauli exclusion principle.) As far as I know the experiment hasn't actually been done, though.

Those cosmic rays are an absolute gold mine of particle physics potential. Some of them easily exceed any accelerator we could build without going solar system scale.

Edit-the best I can think is have every satellite built with a particle detector as a standard. No idea if those could be scaled down.

Edit2-seems doubtful.

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.

We also haven't verified the absorption spectrum of every atom on earth. All science is extrapolation.