Quote Originally Posted by Pandadice
i don't remember it being exactly like the byakugan, but if it has those attributes and then some, i can see why the uchiha were so popular, while the hyuuga wasn't.

http://www.onemanga.com/Naruto/12/06/

okay, so it can actually read the jutsu, so that makes sense that he could tell she was under genjutsu.
"Read the jutsu" is just a quick way of explaining that the Sharingan lets the user copy the physical motions as well as seeing the flow of Chakra in order to properly use the jutsu.

Obito says he can tell she's under a Genjutsu because her Chakra flow is disturbed, but he's really just guessing. All he can tell is that her Chakra is different than his or Kakashi's.