apacolypz

Mainly due to AO's eye was he able to see the color and difference in Danzo's chakra. Him being a sensor helped but his eye was the factor in that case.

Also I could be mistaken but I was sure AO said something of recognizing the color in Shisui's eye. He recognized it because of their past battle. If assumed Obito already had another Sharingan wouldn't that eye have a different chakra color as well? Obito from a color stand point would show three chakra signatures.

http://www.mangapanda.com/93-22827-1...apter-459.html

In Naruto's case if you're a sensor and you can't tell the difference from a really warm chakra vs a really immense dark chakra...than quit your job...Her comment was just that Kyuubi's chakra was similar (sinister) to that of Sasukes. The main feat Karin had in that bout was noticing the chakra levels within Danzo's jutsu. She only knew of the sharingans because she counted them, and she was unaware of Hashi's cells. Like I said her help came when she noticed the number of seconds it was before Danzo could use Izanagi again.

Going by the actions of Obito...Minato's best assumption was Madara.