"Jace?" "Yeah?" "How did you know I had Shadowhunter blood? Was there some way you could tell?" The elevator arrived with a final groan. Jace unlatch… - Cassandra Clare

"Jace?"
"Yeah?"
"How did you know I had Shadowhunter blood? Was there some way you could tell?"
The elevator arrived with a final groan. Jace unlatched the gate and slid it open. The inside reminded Clary of a birdcage, all black metal and decorative bits of gilt. "I guessed," he said, latching the door behind them. "It seemed like the most likely explanation."
"You guessed? You must have been pretty sure, considering you could have killed me."
He pressed a button in the wall, and the elevator lurched into action with a vibrating groan that she felt all through the bones in her feet. "I was ninety percent sure."
"I see," Clary said.
There must have been something in her voice, because he turned to look at her. Her hand cracked across his face, a slap that rocked him back on his heels. He put a hand to his cheek, more in surprise than pain. "What the hell was that for?"
The other ten percent," she said, and they rode the rest of the way down to the street in silence."