how does mcmurphy know chief is not deaf?

McMurphy suspects that Chief can hear, and when he gets the opportunity to call him on it, he doe it in a humorous way. While appearing to lean over to whisper something to the man, McMurphy stop and sings him a song (from the south) about spearmint chewing gum (something Chief love); at first Chief gets mad............ then he takes it how it was meant and laughs. McMurphy has proven his point.