I think the OP means they can't legally access this information other than by getting you or your previous employer to disclose it.
Most large companies won't give out your past salary information, so if they don't force you to provide proof, it's not very likely they will be able to verify it.
>via background checks
Background checks are normally done via a 3rd party background check company, and they don't have access to this kind of data.
When I was hired by Merrill Lynch, in Dublin, they asked for my bank statements for the last ten years in order to verify that I was indeed making what I said. (The only annoying thing was that I had to scan 75 pages... boooring.)
In the US, this is a legal minefield and when a potential employer calls a past employer for references, that past employer will very often not reveal anything more than "We can confirm that this person was employed by us from date1 to date2" and that's it.
They will not disclose your salary, what you did, your organization, who you reported to and who reported to you, your performance reviews and anything else of that nature because all of this extra information is grounds for you to sue them later if you don't get the job you're currently applying to.
The only way The Work Number has access to your salary information is if your employer has given it to them. If your employer is willing to give that out then yes a hiring company can verify your salary.
We are back to my original point that the only way a hiring company can verify salary data is by asking your employer (or having 3rd party ask for them) or forcing you to verify it.
Why is it that "What are you making are your current job, we will verify it." is acceptable but "What was this companies net revenue last quarter" is not. The game is so clearly rigged.
The company bears far less risk than the employee. The company can easily fire you if it doesn't work out, whereas most employees can't suddenly leave a job that treats them badly.
So employees can ask lots of questions about company financial health, as part of trying to reduce the far larger risk they are taking.
Meanwhile, a company doesn't need to know your salary history to make an informed opinion of whether they want to hire you and for what amount of pay.
That's true but far more than 1% of employees are employed by publicly traded companies. Most of the 27 million businesses have no employees at all--there are only around 5 million businesses with employees.
Quick searches reveal this is a popular question, spot checks of the answers agree that it's not illegal and quite common.