One of our most closely-guarded pieces of personal information, we nonetheless are obliged to divulge our Social Security numbers several times a year. You want a job? Hand it over.

A client once approached us to see whether we could run a quick background check on domestic helper he was thinking of employing. She cheerfully offered references, photocopies of a foreign passport and green card, plus a social security number. Our conclusion after about an hour? Her SSN was probably faked, and her green card probably was too.

How did we know without picking up a phone or looking at the document? Anyone with a SSN issued after June 25 of last year got a randomized number. There’s no way for us to tell on our own whether it’s valid or where it was issued. Employers who want to find out if a prospective worker has a real SSN need to use the Social Security Number Verification Service here. You need to register, and there’s a time lag.

What’s nice for anyone else looking at SSN’s issued before June 25 of last year (as our subject’s supposedly was) is that there used to be a system employed in issuing the numbers that could be helpful in figuring out where a person may have lived and when the number was issued.

The first three numbers related to which state the number was issued in. We often find it helpful to know that if someone we’re investigating claims to have lived his entire professional life in New York, his SSN issued in Michigan or Texas gives us a clue that for a time, anyway, he was outside of New York. For someone born in the U.S. that may just mean that he was with his family for a year when he was 12.

But for a person who grew up abroad and got a SSN when he was 35, it’s nice to know where he was when he got that number. For one thing, when checking for a criminal record you get a second state to look at since there is no such thing as a single, national criminal records check available to people outside of law enforcement.

The second two numbers of the SSN are called the series number, and we use these to figure out when the card issued. This was also how we figured our domestic helper was probably faking. The series numbers were not issued sequentially, but were issued according to a known system. If you know the system and when someone’s number was issued you can see whether the number matches the real SSN series issued in that month for that state prefix. Just check the “high number” list).

In the case of our domestic helper, she had a number that should have been issued several years earlier than her arrival in the U.S. Our theory was that she copied part of the legitimate number of a relative who had a good SSN, but that relative had been in the U.S. a lot longer.