Here’s a stained glass window from St. Joseph’s Cathedral of Saint Peter. It’s very easy to tell it’s him, because of what he’s surrounded by. Check it out. He’s holding keys, there’s a papal tiara behind him with a crosier, he’s surrounded by palm branches, which symbolize his martyrdom, and also St. Peter’s Basilica there! Gosh! I love you symbolism. 

I’m not sure what he’s holding though. Is it the epistle(s) he wrote?