imo the hint was in the first chapter.
where the boys are discussing the height of women in tall girl themed bar and say that they can't be taller than akiniba since he's on a taller side.
his friends might already know he's trans and that's why they asked akiniba, because he used to be a "woman". so they can get a ballpark for a "tall girl".
just my guess.
there are also small comments by akiniba himself where he says that he feels like himself talking to women from that bar, etc.