I imagined a great many ways this conclusion could be reached. I like this one best. For one thing, it didn't require an avalanche of dialogue. You know me by now if you've been reading the start: I am QUITE capable of being verbose.
Of course, this way gave little opportunity for readers to guess that they would consider this possibility. The only solid clue was in panel five, and the conclusion to that clue was in panel six. So, ah... If you managed to guess that between panels, give yourself a congratulatory high-five?