chapotraphouse

In we have local chambers of commerce, where business owners collaborate on setting wages across a local area. I'm guessing landlords use those statistics as well

My lease renewal had a required "employer" section that includes contact information for said employer. That's probably one way.

It's legal for landleeches to ask for income source and amount. Most do.

As far as I'm aware:

A landleech has no way of seeing your wage after you give the initial application with proof of income. I've lived in many places and signed multiple lease renewals all while changing or losing jobs many times. The only thing that mattered was that first application, none of the lease renewals required ongoing proof of income.

In the U.S. there's a government tax/income database (presumably the IRS) reachable over the internet where all income and employers are logged and searchable. I believe state welfare agencies and related (such as subsidized housing) get full access to this treasure trove but I don't know if private business and individuals can.

Generally, government jobs will have openly disclosed wage schedules given that they're publicly funded. A government employee can't really hide their income if someone wants to know it.

Knowing a non-government employee's precise wage at any point in time or being aware of a raise would require direct communication with the employer, so if someone like a private landleech is privy to that kind of info, there's probably some very illegal and/or unethical shit going on.