How he would know that I can't be sure. Why is pretty obvious, if indeed he is not Obitio: to screw with him.
I see the reasoning behind the chapter now. It had been a while since Tobi's powers were briefly explained and if you pay attention, Tobi has been doing some deliberate misleading with his powers if they work this way.
Originally posted by AuraAngelsakura is kakashi level in intelligence 😛
http://www.mangapanda.com/93-400-10/naruto/chapter-395.html
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tobi's a grave robber of obito's eye (and maybe left over flesh), we already saw him do it with nagato.