In the same way as the author: by imagining it. The question is, why do you choose to imagine only the two scenarios in the strip, and reject the legitimacy of imagining the third?

If I can imagine being at a switch deciding whether a train will kill five people or one, I can also imagine everyone getting off the train and the train derailing where it doesn't kill anybody. But that would defeat the whole point of imagining the train in the first place.