Kingsblood Royal by Sinclair Lewis

He crossed the lobby of his hotel in Minneapolis with his eyes rigidly held on the black-and-white marble of the floor, irritably noting that it WAS black and white, careful as a drunk who betrays himself by being too careful in his gait. He was wondering who might be staring at him, suspecting the Negro in him. Wilbur Feathering, who was a food-dealer in Grand Republic but who had been born in Mississippi, frequently asserted that he could catch any "Nigra" who passed for white, even if he was but a sixty-fourth black. If Wilbur did detect it, he would be nasty about it.

Right in the center of the lobby he wanted to stop and look at his hands. He remembered hearing that a Negro of any degree, though pale of face as Narcissus, is betrayed by the blue halfmoons of his fingernails. He wildly wanted to examine them. But he kept his arms rigidly down beside him (so that people did wonder at his angry stiffness and did stare at him) and marched into the elevator. He managed, with what he felt to be the most ingenious casualness, to prop himself with his hand against the side of the cage, and so to look at his nails.