I can't prove it, of course. But if we suppose that the hypothetical non-humans work within our physical reality, and are subject to the same physical laws, then I'd suggest that there are only so many ways to, say, float on water or generate artificial light; any vehicle would have to be of a proper scale to carry them; they'd probably have to carry heavy objects, and we could probably recognize the characteristics of a carrier; their sensors would likely detect the same sorts of things that ours do; they might very well generate and use electricity; and so on.
If we turn our attention to living things, the "solution space" for manipulating cellular, DNA-based life would be constrained by the properties of the organisms being manipulated.
I'm saying you can't test for it the way you test for human design, because you don't have the tools (facts and inferences) that you use to determine human design.
All that really means is that any test based on such an assumption would not be successful. However, the "human approach" test would still have explanatory in caseS where there was a strong enough similarity between human and non-human approaches.
The problem is, the crows very neatly refutes your argument, insofar as nobody recognized the artifacts as tools in and of themselves - the only way anyone knew that the crows were using tools was by observing them in the act of using them.
Yes -- although one could say the same thing about the early efforts of stone-age humans. But if you'll recall, the question was whether or not we have a catalogue of non-human products against which we compare our own works -- in the context of whether a solution space is constrained by the physical characteristics of the problem at hand. The lesson of the crows is that those non-humans used their stick tools in ways that are recognizeably similar to what we would do.
But no human, other than a psychopath, would design the ecosytem as it is. Humans, aside from criminals, do not design machines that eat other machines. And it's quite possible to have living things that don't eat other living things.
It's true that humans build war machines and destructive machines, but an entity capable of designing life, and which designed what we see, would be criminal.