How could Tosen know that Kenpachi was smiling? When Tosen fights Kenpachi in the Rescue Rukia arc, Tosen uses his bankai to combat Kenpachi.

A while later, Kenpachi figures out how to defeat Tosen's bankai and smiles. Tosen somehow knew that he smiled (Because we heard his thoughts: "He's smiling?")

However, Tosen is blind. How could he have known?
When Tōsen attacks with Enma Kōrogi, He is not affected by it as anyone else (he can still smell and hear).
So when he asked why Kenpachi laughs it was because he could hear him laughing.