Depends. If the person works for a state or government than it may be listed in public records. When I worked for the state of Texas anyone could look up my salary. Someone's income can end up on court records if they are sued. Of course anyone who can legally obtain someone's credit report or income tax return also can find out. We really have very little financial privacy any more.
Unless you have a court order requesting that it be turned over to you, you can't - that's confidential and an invasion of privacy.