Now a spaceship accelerating or decelerating in relation to our system at a significant chunk of the speed of light -- detectable from the heat signature coming from its radiators -- that, to me, would constitute uncontrovertible proof. Or perhaps a working quantum computer with a million qubits or some such number that puts it centuries ahead of what we can build today.
That's just two types of proof off the top of my head. Of course, there may be others that I cannot think of due to my limited imagination, but that would generate near-universal acknowledgement once they became known. Isotopic ratios would be irrelevant here because they either cannot be determined (the distant fast spaceship) or the device may have been built with alien know-how but local materials (the powerful quantum computer).
Of course any artefact determined to have such ratios as are not found in our solar system, if presented, should count as prima facie evidence. Like everything, it would still need skeptical review and scrutiny.
Witness statements, whether sightings or abductions, buried memories uncovered by hypnosis or communications via ouija board, are not low-grade evidence, they have ZERO evidentiary value. Sorry.
I used to say that non-blurry, crystal-clear footage might count for something, but in this age of "deep fake" technology, that is no longer the case. Sorry again.