The most unlikely part of this dilemma is that he asks his cat if he ever hums. Everyone gets songs caught in their head. Everyone knows at least one cat food jingle, probably the Meow Mix song, because it is diabolically catchy and hilarious. I don't know if Alpo used to have a jingle of any note, but maybe Garfield also remembers that one from his endorsement deal back in the day. But Jon has been under a daily barrage of un-catlike behavior from Garfield, and constantly treats him as an equal. I love those moments when the unreality of this world becomes such a matter of course that Jon can just assume that Garfield has the ability to hum.
Mechanics of Garfield Thought Bubbles: Garfield is usually able to prompt Jon's disgusted sideways glance through meaningful silences and body language. Today a lack of verbal response and movement would indicate tot he casual reader that today violates the cardinal rule that Jon cannot hear Garfield's thoughts. Among the possibilities is a subtle version of: "Did I... really just ask my cat if he ever hums?"