Take a story's temperature by studying its tone. Is it hopeful? Cynical? Snarky? Playful?
The story starts off in an almost journalistic way, as the narrator makes some very general observations about the rise and fall in the popularity of hunger artists. But this almost journalistic voice can't be taken at face value. The thing about Kafka's writing is that he seems not to know that he's making an ironic comment, but he's really totally aware of it.
(Digression: It is a bit like dealing with someone who makes no attempt to conceal the fact that he's stealing a cookie from the cookie jar. He isn't really inept at stealing – he wants to be caught to show that yes, he took a cookie, and yes, he dares you to punish him for it.)
For example, the narrator tells us that the artist feels "queasy" and "nauseous" when he sees his first meal at the end of his forty-day fast. Up to this point in the story, we may take the narrator at his word and think either that, like many people who have not eaten for a long time, the artist's body needs to gradually take in food, or that the artist is so committed to the demands of his art that he only very, very reluctantly takes the food.
Later on in the story, though, we find out that the artist actually started fasting because he couldn't find any food he liked. The nausea we read about earlier had absolutely nothing to do with fasting or artistry: the guy is just a super-picky eater. The joke is on us, as it is on the artist's fans. The narrative is constantly playing with our assumptions in this way, at the same time that it passes itself off as matter-of-fact and objective.