Nearly all job applications here in Burgerland have an agreement at the end where by signing it, you give them permission to verify all declared dates, places, and sometimes job titles and salaries of past employment, in addition to the usual criminal and occasionally credit checks. You can usually tell it's being done because you get semi-interactive email updates on its progress and sometimes a pause and request for more info.

So how does some random employer even get info like this to begin with? From shit like LexisNexis, The Work Number, and various other harvesters who maintain lifetime databases on workers, fed in part or in whole by networks of affiliated employers who submit employee histories to them. They are the snitch services who cater these background checks to employers.