Charles Ellicott Commentary Genesis 42:8

"And Joseph knew his brethren, but they knew not him." — Genesis 42:8 (ASV)

Joseph knew. — As this is twice repeated, some suppose that Joseph (Genesis 42:7) had only a suspicion, from their dress and appearance, that these Canaanites were his brethren; but that when they spoke the Hebrew tongue , every doubt was removed. They would not recognize him, as he used the Egyptian language, was dressed in a white linen dress, and being only seventeen when sold, had during the twenty years of separation changed in appearance much more than they had.