Yeah, I guess the time machine in her room gives it away for the viewer, but he obviously spots the ring in the picture. It's hard to see because of the subs, under the S:
Well if Kuon had told him the truth in the first place! I guess that's why they never actually kiss in the opportunities that they had in this episode because the writers knew that might weird a lot of people out when the truth was revealed.
The problem I have with this episode is that he should have run into his old self as well! Normally when the show's had some outrageous plot it has been explained away by something that actually approaches sense, but it didn't do that this time.
This is something that really annoys me about anime but wasn't as prevalent (in mainstream anime) in the past. I think a lot of people who make anime are under the impression that it doesn't matter if you're not blood related, but these days it doesn't seem to matter if you are. I expect these people haven't actually got siblings though...because...you know? Bleugh