After seeing some of the differential wear patterns on keyboards, I think a cool spy movie technique would be to spray something (light glue, maybe a light glaze of sugar, etc.) on the keys so that the first use creates differential wear.
i.e. you can see which of the 4 keys has fingerprints after one use.
no subject
i.e. you can see which of the 4 keys has fingerprints after one use.