Why do you ask?
It is possible, but not likely. They could, of course, hack into either the employer's or IRS files. They could hack into one's personal PC and open their Quicken or Money file and look at deposits. They could peek at a paycheck while visiting. There are any number of ways, they could find out, but it wouldn't necessarily be easy.
I remember, when I first starting working full time, I was in a manager training programs. Salaries were proscribed based on your progress in the program. So everyone knew what everyone else's salary was. In my next job, it was not like this and I, innocently and in the course of a conversation, mentioned my salary at a lunch one day. I learned then, that you don't discuss your salary with co-workers.
