"Current system is "voluntary"."
I just went to Google and typed in "SSN check". I got back 15 secuirty companies and 1 listing for the SS administration. The SS admin site told me my SSN was confidential but didn't give any option to check it. Maybe I'll try their main website.
All they can do is match name with number. They will not tell you your sources of income so you don't know if someone is using your number until the IRS wants lots of back taxes and fines for not reporting all your income on your tax return.