paternity-test
Explanation
The Joke
A doctor tells a man that "the paternity test is complete" and that "I'm afraid you're not a father." The man is shocked ("What?!") and protests that they didn't draw blood or anything. The doctor's response: "You're wearing cargo shorts with a phone holster." The man's rebuttal -- "They were ten percent off!" followed by "Nooooooooooo!" -- only further proves the doctor's point.
The Humor
The joke plays on the double meaning of "paternity test." Instead of the medical DNA test to determine biological fatherhood, the doctor has performed a cultural assessment of whether the man qualifies as a stereotypical "dad." The verdict -- based on cargo shorts and a phone holster -- is that he IS in fact a dad, which is the twist: his protest of "What?!" suggests he thought the test would prove he wasn't a father, but the cargo shorts are the ultimate proof that he is.
Actually, re-reading the comic, the joke works the other way: the doctor says he's NOT a father, despite wearing the ultimate dad uniform (cargo shorts with phone holster). His outraged defense that they were "ten percent off" -- a classic dad obsession with small discounts -- hilariously contradicts the test result, suggesting the test must be wrong because everything about this man screams "dad."
The humor relies on the well-known stereotype that fathers adopt a specific, unfashionable style (cargo shorts, phone holsters, obsession with bargains) that becomes a kind of cultural uniform.