From Gestalt

Craftreading is the art of seeing the hand of a witch in the evidence of their work or hexcraft. Because any hex, sigil, or fetish is meticulously prepared and woven over the course of anywhere from minutes to years, and there are many paths to any desired result, the personality and specific techniques of each witch make their way into the so-called grammar of their fetching. Any witchblood with sufficient experience will be able to see this signature, each unique as a fingerprint. In order to know who it is as opposed to simply knowing the signature, the craftreader would have to have encountered their work before and know the identity of the crafter of the previous work, as with an actual fingerprint.