Arrowheads are easy to distinguish and are almost never mistaken for geofacts. The trick, as I can attest from actually attempting to do it, is to distinguish stream-shaped rocks from human shaped. This is not done by some kind of criteria-based design/not-design algorithm, but by a simple test that looks at the angle of the alleged cutting face, and assigns everything under that angle to the geofact pile, and everything above, to the artifact. Note again that this is based on and confirmed by empirical experience with actual designed artifacts.
The only way to infer design is to go from some known artifact to an unknown one of the same category. Unfortunately, technologies, the usual comparison, will not work, because they are poor analogues for living systems. We have no analogues, and thus, are not in a position to infer design.
Further, living systems are apparently ad hoc -- which fits evolution -- and not designed well at all. The human ear has three bones that are jawbones in older animals. Why modify like that? Why not start anew? Natural designs are often inferior to what any rational designer would do when starting from scratch with unlimited materials and power.
Another reason we need not infer Design is because we already have a model, natural selection working on various changes in the genetic material, that explains the variety of life on earth. There is no need to turn to a Designer, when that powerful model is available.
Lew Alton
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