Heh, your flip remark leads to an interesting observation: I think the most obvious signature of "abstract intelligence" is suboptimality. An alien would look at an agricultural grid and wonder, what possible natural process would divide something into squares rather than hexagons? Even without knowing what they are or how they form, it's hard to imagine anything that would prefer such a high energy configuration. Such stupidity implies a system oddly bounded in its ability to optimize, which implies a feeble biological brain trying to do something "on purpose" rather than a natural process.