@Nolonar Well, waiting inside a chair was one clue, though not that definite (maybe it was quirky security guard), but intimacy with main character was dead givaway: there is not a single chance for non-prince to flirt with VIP on the first date.
There are also hints in that prince is not showing himsel in front of people and prince being perceptive person (explaining flirt with a guy and hinting the cat might be out of the bag from the get go).
The final and decisive clue was the boner. I mean, with enough stimuli you can probably do that to a man, but this is manga, so it was definitely a woman.