[Magnus’] warlock marks are his yellow-green “cat-like eyes” with vertical pupils.
I can't wait to see Crowley's eyes, either! Part of me, though, really hopes we DON'T see them until his glasses are knocked off in the burning bookstore. I think this was when they were revealed for the first time in the book, and it's super dramatic and badass. (I'm sending this as an ask bc the post you mentioned his eyes in was already pretty long)