At around 8:20 in Episode 4 of The Quintessential Quintuplets, Ichika asked Uesugi,
Hey, don't you want to find out if girls truly wear nothing under their yukatas?
Uesugi said,
I already know.
Uesugi was concentrated on study and had no interest in love or the like. Therefore, no girl would ever tell him that and his sister is not old enough to wear one. How did he find out the truth?