Without something esoteric such as DNA that bears no resemblance to human DNA, I know of no possible way to examine an inanimate object and declare it to be "non-human" in origin.
Well, to use JMartJr's phrase and play Devil's Advocate,
Grusch does claim (without evidence or any expansion on the subject) that alien "pilots" have been found.
Unless permanently cocooned in spacesuits, organic creatures having any similarity to large multicellular Earth organisms would presumably leave organic traces (microorganisms, DNA traces or their equivalent).
Ratios of isotopes in any metals found from an ET artefact might be expected to differ from the isotopic ratios found in terrestrial metals-conversely, the fact that the Earth and Moon have similar ratios of oxygen isotopes is taken as evidence for the giant impact hypothesis (that much of the Moon's material was once part of the proto-Earth),
"Oxygen Isotopes and the Moon-Forming Giant Impact", Wiechert, U., Halliday, A.N. et al, 2001,
Science Vol 294, Issue 5541
https://www.science.org/doi/10.1126/science.1063037
I suppose it's possible that X-ray diffraction studies might demonstrate that a manufactured material had a molecular structure that was more complex than something we could currently produce (or are likely to produce in the near future).
ET technology/ materials
might display properties that are only theoretical to us- unipolar magnets, high-temperature superconductivity, something that blocks neutrinos, Pringles tubes that protect the crisps from being squashed by shop workers, etc. etc.
If a recovered craft was a crewed vehicle which clearly required a pilot or whatever, ergonomics might be a clue, e.g. a cockpit position with complex controls that can only accommodate a creature one metre tall, controls that require four hands simultaneously, that sort of stuff.
Maybe Grusch was told about a recovered bumper (fender) sticker, "My other saucer's a mothership".
I think if we were to find an alien craft in the sense of a crewed "lander", it would be clear what it was.
My suspicion is that, if it exists, a reasonably sophisticated ETI capable of traversing interstellar space and with an interest in surveying Earth (for whatever reason) could probably avoid detection if it wanted, e.g. through the use of arbitrarily small (maybe nano-engineered) low-observable "probes", or simply by stand-off observation.
