Cause Minato has markings on his hands and feet similar to that of Biju mode, along with the spiked up hair. Don't quite get why the yang chakra holds the consciousness of Kurama, you would think the consciousness would stay with the spiritual chakra that deals with the mind. I thought the consciousness was simply split but apparently not if the people's responses is anything to go by.
Minato is definitely in BM, naruto's and kurama's reaction proves it.
Also, for all those thinking Kurama consciousness isn't in Minato, consider Orochimaru. he was inside Sasuke, Anko, and other people at the same time.
Itachi sealed Oro, yet he's still alive and continuous!