I don't know that, and neither, I suspect, do you. We know that human processes are constrained, of course - some of the proposed candidates for Grand Universal Designer are not, shall we say, posited to be quite so limited. In which case, there's no reason to expect commonality, nor does the lack of it count as evidence against a presumably ineffable designer ;)
What would be bad science would be to simply assume we couldn't recognize non-human design, and not try to test it at all.
I'm not saying you can't test for it at all - 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. That's why the two cases are not at all analogous - to do it that way, the way you examine things to decide if they're human artifacts or not, you need knowledge about non-human artifacts that you don't have. Worse still, you need knowledge about non-human artifacts that, thus far, you can't get.
Or you have to find some other way to test. Dembski thinks he has another way - I think it's complete bunk, but there you go.
For example, it's known that crows and other animals invent and use tools. True, they're not terribly sophisticated tools.
I believe you've mentioned the crows before. 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. Do you expect to be able to observe the Designer in the act of designing, so that you may recognize his handiwork later? ;)
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.