Just for the record, btw. (it's a well-established finding, not just a personal "belief") here's one of many example papers where citations to the previous research findings on ability to perceive sex is stated:
https://www.sciencedirect.com/science/article/pii/S004269892200133X
For example, from the introduction, on human faces:
"Nevertheless, among the most reliable information that a human face provides is the sex or gender of its owner. In a sociocultural context, there are several conventional signals that indicate a person’s sex through their faces, such as hairstyle, facial hair, clothing (hat, bonne), accessories, makeup, etc. However, research shows that even when all these traditional cues are removed, people are still able to identify sex with 96–98 % accuracy from facial stimuli (Bruce et al., 1993; Bruce, Ellis, Gibling, & Young, 1987; Saether, Van Belle, Laeng, Brennen, & Øvervoll, 2009). "
As I say, it's really only a small minority of people who do not seem to have the ability to correctly sex others. With other cues that you'll get in the real world apart from faces, such as gait, build etc, it becomes even more obvious. It's purely physical, and not to do with things like hair or outfit, as the above notes.