I wrote a lengthy comment on just that, but scrapped it because it became rambling.

An outsider could indeed tell them apart by scanning for exact structural correspondence, but that seems like cheating. Peering beyond the veil / opening Clippy's box is not allowed in a Turing test scenario, let's define some p-zombie-ish test following the same template. If it quales like a duck (etc.), it probably is sufficiently duck-like.