I love that when it comes to Ran he just knows, because she's the only girl he's interested romantically but with Ai he's cluless because he isn't attracted to her or loves her.
Hehehe, How can he be so observant with the girl he loves meanwhile he doesn't notice his friend has a crush on him? Sorry Ai, you're an amazing friend but you can't awake Conan special ability because he only uses it on Ran and the couples he helps!