It didn't take long for Serendipity to work out Azrael was a demon. He learned from Lucifer how to put on a show for the mortals, and he learned the art of glamours well. But Serendipity isn't mortal, and she knows what to look for. So, yes. She knew.
She thought, fleetingly, about refusing him. But she so rarely gets clients who know what she is and what she can do. Why turn away the first in a very long time, just for being a demon?
Besides, wearing the glamour, he really did look good. So she took him in.