This thread isnt an original but come to think of it.
Remember this (i got remembered these exact lines from the Anime)
"You are weak brat! Muhaha. You should be thankful to me and for the time when i was sealed. By the FOURTH HOKAGE!" (with alot of anger saying that..red glowing eye
)
This was said by the Kyuubi when Naruto pretty much broke his neck by Sasuke and Naruto started to grow a tail. Sasuke vs Naruto on the waterfall.
Now i think the Kyuubi would know if he was sealed inside another persons body or that of the fourth.