What clue made it possible for the the man in the yellow suit to recognize the tuck's?

The man in the yellow suit recognized the Tucks primarily by their unique and unusual appearance, particularly their agelessness and distinctively old-fashioned clothing. He had previously encountered the Tucks and noted their peculiarities, which included their lack of aging and the striking, almost timeless nature of their features. This familiarity, combined with his knowledge of their secret, allowed him to identify them.