One character has recently lost their virginity (or a pair of characters has recently lost their virginity together). This fact is immediately obvious to yet another character (usually, but certainly not always, an older woman), despite there being absolutely no outward sign or obvious reason to think that. The characters aren't acting cute together, there's no tell-tale messy hair or mussed clothing, nor even the subtle aroma of "just had sex".
The other character merely looks upon the new non-virgin, as if using Virgin Vision as a superpower, and they know. (Women's Mysteries, perhaps.)
Compare to Did You Just Have Sex? (where other people know because the characters are giving away clues) and Virginity Flag (where there is some obvious means of telling who is and is not a virgin).
As an inversion, in the French comicLe Chant d'Excalibur, everybody can apparently detect that the heroine is still a pucelle, and will mention it at the first occasion.
In Say Anything, Lloyd Dobbler's friends can tell that he and Diane Court "did the deed", despite Lloyd's "I admit nothing" disclaimers.
Inverted in The 40-Year-Old Virgin, where the 16 year old daughter of the main character's romantic interest can tell he's lying about having had sex and is still a virgin. When he asks how she can tell, all she says is that she goes to school with tons of boys and can somehow "just tell" who's had sex and who hasn't.
In the Morganville Vampires, Claire fears this trope. Sure enough, Eve barely looks up and realizes what happened.
The first chapter of Alina Adams figure skating mystery Death Drop starts with the researcher heroine listening to commentator insisting "You can always tell a virgin by the way she skates." She thinks the non-virgins skate better. She turns out to be right in her assessment of the skater they were watching.
In Blue Smoke by Nora Roberts, Reena's mother is able to detect instantly that her daughter has lost her virginity when Reena arrives home after doing the deed. The scene is retained in the made-for-TV movie based on the book.
Not a virgin example in The Lost Fleet: Dauntless, but the first time Captain John Geary has sex after his 100-year stint as a Human Popsicle, the titular ship's captain, a young woman, immediately knows it, even though he's doing his best to act normally. When asking his sexual partner, she replies that this is a "need-to-know" kind of information. He interprets it as Women's Mysteries. To be fair, though, he does admit to himself that this is the first time he felt himself without "ice" inside him, so it's likely he was giving off subtle signals.
After Donna and Eric finally have sex in the third season of That 70s Show, neither one admits anything to the other characters (despite Eric really wanting to do so). When Donna goes to talk to Jackie, Jackie see her as having the words "I Had Sex" written on her forehead, and Jackie immediately rushes her away so they can talk about it. When Eric goes to talk to Bob, Bob sees Eric as having the word "Guilty" on his forehead, inidcating that Bob is suspicious, but Bob makes no sign of knowing what happened until he officially finds out a few episodes later.