When viewing artistic depictions of Christian martyrs, it is often very easy to recognize the way in which the saint was killed for the faith.
Many Christian artists over the centuries made it a point to visually represent the means of a martyr’s death, seeing in it a sort of “badge of honor” that proves the saint’s holiness.
Take for example St. Bartholomew the Apostle. There is a tradition that states he was flayed alive, and so many depictions of him have St. Bartholomew holding a knife and his own skin. One of the most realistic versions of this is a statue in the Duomo in Milan.