An Echo in the Bone

How does Ian know that William is his cousin when he meets him in the Great Dismal and gives him Jaime's bear claw?

Touchstone: An Echo in the Bone.

By the tattoo on his forehead?

Surely because William is very like his father and sister?

By the pricking of his thumbs?

Like the bible says: Greater love hath no cousin than this, that a cousin lay down his bear claw for his cousin.

mmmm, bear claw.