Well, as for concrete clues, all I can think of would be that when Kellhus speaks to Aurang, he confidently says to him that he knows how his actual physical body is really in the Ark, with the Consult sorcerers making it possible for him to inhabit the Synthese. If Kellhus was lying, I feel Aurang would have been quick to point out how little he actually knew. Then again, there is the chance it was just a cat-and-mouse statement, with neither knowing which role they were truly playing.
That, paired with the idea that Aurang was concerning himself with the Holy War and the South, while Aurax scoured the North.
Keep in mind that they are twin brothers. I think it is reasonable that they should look alike. The chapter does speak of "cancerous growths" that I don't recall ever being described on Aurang, as well, IIRC.