Quote Originally Posted by animus
Is it me, or does Itachi ALWAYS have his sharingan on. The silhouettes with them putting chakra into extracting the Bijuu from Gaara, show him with Sharingan too. Maybe, it's just like a plot device to allow people to recognize his silhouette. But, man he's got it on like all the time so he must have some insane amount of chakra.
And maybe that's how Kakashi knows Itachi's eyesight is deteriorating? After all, Kakashi's Sharingan is always on as well. (Sure, he keeps that eye closed and covered most of the time, but it's still on.

Of course, this begs the question of just how much Kakashi knows about the Sharingan, period, as well as how he learned it. (Trial and error? Classified Uchiha documents? Special training from the Uchiha clan?)