Shadow mismatches true form

Peryton

Its most striking feature, however, is its shadow, which takes the form of a human silhouette rather than reflecting its true physical appearance.

Kumiho

The Kumiho can typically be exposed by its shadow, its reflection, or the fact that it retains a hidden fox tail or fox-shaped ears even in human disguise; in several well-known tales, a suspicious husband or monk uncovers the creature's true nature through one of these tells and it is driven off or destroyed.