Subject: Re: A Funny Thing Happened In Nebraska
A one-time check that a given name matches a given SSN, and the SSN is valid, i.e. not a dead person.

If valid, then it is fairly quick (seconds/minutes).

Is it a *unique* SSN number not used by anyone else? Prior employment using that SSN?

Migrants do not usually have a SSN, they would have a work visa and maybe TIN (again, another unique ID).