A group of four girls led by Kaori Hanazono begin to suspect that Hibari may be transgender, but Tsubame takes Hibari’s place during a health checkup, effectively quelling any suspicions.
The giveaway is that Hibari has blue eyes, but his sister Tsubame (the girl in the video) has brown eyes.
Actually, all the the girls in this video are AFAB. The girl that is undressing is not Hibari-kun, but his sister Tsubame. Source: https://en.wikipedia.org/wiki/Stop!!_Hibari-kun!#Plot
The giveaway is that Hibari has blue eyes, but his sister Tsubame (the girl in the video) has brown eyes.
Well damn, guess the trick works on adults as well as it does Japanese schoolchildren.