Quick searches reveal this is a popular question, spot checks of the answers agree that it's not illegal and quite common.
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.
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.
I'm going to be more careful next time. :D
Yes they do. There's a company owned by Equifax called The Work Number that collects such information.
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.
[1] http://www.forbes.com/sites/sageworks/2013/05/26/4-things-yo...
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.