Originally Posted by ShaolinLueb

i don't follow the Anime anymore cause too many filler episodes introducing nonsense like Naruto dancing with a toad. I thought it could only sense chakra.

http://www.onemanga.com/Naruto/459/

this is going to get good again soon.

Originally Posted by http://naruto.wikia.com/wiki/Sharingan

The Sharingan's second-most prominent ability grants the user an incredible clarity of perception, allowing them to easily recognize genjutsu and different forms of chakra, though not to the same extent as the Byakugan.

The third of the Sharingan's powers is being able to see the chakra flow. The Sharingan can tell if a person is under a genjutsu because the person's chakra flow would be irregular.