Manga: I'd say volume 3/4, when she asks him for help to find her brother. Me thinks the first time he blushes is when they're stuck in the maze and Sakura asks if he gets lonely living によって himself. But of course, he doesn't realise his feelings until much later.
Anime: It's a little harder to pin point, especially since it's easy to check the マンガ (since I own it and such). He actually confesses at the very end of the anime, but it is Sakura and gives her response in the 秒 movie (not Syaoran. She already knew his feelings).
As for the crush, it was obvious によって the late twenties (ex: episode 27, he faints when she hugs him). So, I'd say episode 17, when Sakura goes through the test of courage.
I can only help with the anime. I'd have to say it kind of happens gradually. He never really has a crush on her, he just falls in 愛 slowly. It is not one of those things where he realizes it in one episode, it's just kind of slow, which makes it all the sweeter <3
Well, I guess he started to have a crush on her after the Return card episode, but あなた really have to open your eyes to see it. His crush was visible during the Fiery card episode, ( ep 35 ), and によって the end of the 秒 season, he fell in 愛 with her instead. ( Good for him ) But seriously, he's a BAKA to not admit his god damn feelings earlier, so the アニメ would be もっと見る interesting. But... あなた can say it has a charm of its own that way.
