How accurately do humans identify biological sex?

I don't think if these markers were not there one could tell.

It doesn't matter with kids though does it, because they are not mating with each other? That's why post puberty the differences become much more obvious isn't it?

I don't think if these markers were not there one could tell.

On the other hand I worked with someone for months without suspecting she had previously been a he...

When it comes to people who have had surgical and medical interventions it's a completely different matter because they have deliberately taken steps to take on the characteristics of the opposite sex.

It takes a lot for a person to pass as the opposite sex. Most who try never completely acheive it.

In the general population adult humans are very clearly either male or female looking. Children are naturally androgynous so you can only tell through gender markers such as short hair / blue, long hair / pink etc. It's easy to swap clothes and hairstyles on children to confuse all the adults as to what sex they are.

With children I would think very inaccurately

I Googled boys aged 5 and girls aged 5. The boys had short hair, not just short but mini-me versions of the sort of hair cuts adult males have. They were wearing mini-me versions of the (generally) dull clothes adult males wear.

The girls had long hair or, if short, mini-me versions of adult female cuts. Their clothes were much nicer-not just dresses and skirts but nicer, brighter colours.

I don't think if these markers were there one could tell.

People often say that their animals can, some pets who have been mistreated don’t like men for example. If often wondered what a cat or dog is looking for, or is it smell.

Any research studies etc?

There are too many variables to make any kind of study reliable.

With children I would think very inaccurately. I certainly would go on clothes, hair etc until puberty. After that I think pretty well. I was trying to sport the women in the trooping of the colour even though they were all dressed identically I and I think I can.

It would be very interesting and timely research. I don't know of any though.

I remember reading an article about some research that suggested that the first thing you erceive is a person’s gender/sex ( at this stage I dont remember which). Essentially on being given a brief glimpse of a new person, there study showed that people had time to clock whether the person was male or female but didnt always remember quite noticeable other features such as hair colour

On the other hand I worked with someone for months without suspecting she had previously been a he...

We've evolved to be good at detecting biological sex, as a way of ensuring propagation of the species, and of reducing conflict within social groups.
Its why the description 'androgynous' exists. If androgynous were the standard we wouldn't need a word for it. People who looked wholly masculine or feminine would be the outliers that needed a description.

There isn't a single species on the planet that cannot detect their biological sex.