For serious? The top of my kids heads smelled intoxicating for the first 6-9 months. There is still a special scent but not quite the same. And I smell it on other babies, too. It’s like smelling breast milk, but emanating from the Fontainelle
As a teen, I once got heckled by some coworkers. "Wow, are you pregnant? Your face is radiating!" I had accidentally put on too much oily sunscreen that day. I'm a cis male with a beard.
It's not so much an obvious smell as a perceived "sharpness" that feels related to the sense-modality of smell, but is not exactly equivalent to it. I wonder sometimes if it's a pheromone-based phenomenon.
I can often smell when my wife is in a bad mood (her breath literally smells sour). However, it isn’t a reliable indicator (many false negatives).
Although getting serious statistics on the matter is not easy, the estimated numbers can be surprisingly high (e.g: "One in 25 fathers is not biological parent": https://www.theguardian.com/society/2005/aug/11/childrensser...)
If suddenly we discovered an easy way to give a hint you should do a DNA test, it could do some serious noise.
Many years ago my father was once given hospital test results stating he was terminally ill with a heart condition. He knew he was somehow given a different patient's test result under his name when when his family doctor asked him if he exercises (he does) and the doctor said "That can't be your patient record. You wouldn't be able to exercise if your heart was as sick as the test result states." My father is still alive and exercising regularly today.
I wonder if its brains you smell, and a fully fused skull prevents the smell from escaping.