Many men underdress and wear panties and hose under their jeans, or suits. Some men underdress all the time.
If a person didn't change anything else, except put on a pair of pumps or feminine shoes, that would be the biggest factor.
Even without undressing. If a person wore all male clothing but no socks and pumps, people would notice.
I went to the shoe store yesterday and purchased a couple of pairs of women's shoes. Like all other apparel the women's section was probably more than 90% of the store. As I walked up and down the aisles I noticed many male styles with low heel and everything looking exactly like men's shoes in the women's aisles.
Prior to going to the store I had discussed this with my wife. I had told her that it is not right that women can dress in men's suits and wear them and look totally beautiful. They can wear men's work clothing and work boots and remain feminine and look beautiful. Obviously any person that wears clothing should have clothing that fits them. No person would look good in a suit 6 sizes too large. So a woman in a male style tailored suit is still a woman in a man's suit. I believe that a handsome man can still be a handsome man in stylish female apparel.
I plan to wear women's shoes with or without hosiery in the future when it is appropriate. I am going to clean and repair an apartment I own today so today is not a appropriate. But going to the grocery or most other places would be.
I believe that if nothing but shoes were changed, people would identify a male as a crossdresser by just changing their shoes. Obviously a woman could wear the exact same jean's, shirt, belt and everything else and would not be crossing or be considered less feminine.
It it is kind of crazy that every shoe in the world can be worn by a female but 90% or more if worn by men deem them strange.
Shoes are the greatest single identifier.