There are three distinguishing marks that identify a Mayfair witch. The first is being born with red hair. The second is being born with a sixth finger. The third is growing taller than even above-average-height women. For instance, Rowan Mayfair has red hair. Mona Mayfair has a sixth finger. Morrigan Mayfair is very tall. A witch does not need to exhibit all three marks; one is sufficient for identification.
The Mark of the Witch appears in Blood Canticle (2003). For more perspectives on the Mark of the Witch in the Vampire Chronicles, read the Alphabettery entries Mona Mayfair, Morrigan Mayfair, and Rowan Mayfair.