Shirou already had his suspicions when she spoke of being able to break down Rin's bounded field, but when she spoke of making the cooler idea work (with gusto and confidence), it only confirmed them. Not that he felt he was in any real danger around her; she didn't give any hostile vibes. Not that he'd have to use it here (hopefully), but that just meant that if she was a magus, his Reality Marble would have to remain hidden still. This was assuming that she was from the association too...

"Ah, you're a magus too." His tone didn't imply any disdain, more of an observation.