Pure and simple, you don't. There is a link to a list of legitimate companies at the bottom of each page of the forum. These are companies that members of this forum use and have used without issue. But you know as well as I do that your bank has your SSN, your credit cards and likely your cell phone provider have it, any past or present employers has it, your tax preparer has it, your doctors and insurance have it and most likely any school you ever attended has it--and that is just a start. Remember that resumes used to require it and some still do, even if you were/are never hired. All it takes is one dishonest person at any of these places and your information could be compromised. All it takes is a breakin at their offices to get the information. All it takes is injudicious disposal of old records without shredding.
Life is full of risks. If you want to stay absolutely safe you will never leave the house and you will pad every surface in the house, never answer the phone, handle incoming mail or eat anything that has not been sterilized in the canning process. You rather measure risks and where it comes to legitimate companies, that is pretty small.