There are tons of stats that serve as effective proxies for such things: What schools did you go to, what is your name, who are your friends, etc.

So yes, it is EXTREMELY hard.

https://www.reuters.com/article/us-amazon-com-jobs-automatio...

Thats exactly what amazon tried to do here, but because the AI based its decisions on amazon's previous hires (that is, almost no women) and blinded the AI to gender, it instead proxied women for one of a dozen other things such as school, courses taken, etc.