Winchester, Sam.
If you were to go through the collection of IDs in the Impala's glove compartment, you wouldn't find a single one that says this. What you would find is a jumble of badges for the FBI, the CDC, U.S. Marshall, the National Guard, State Trooper, Homeland Security, and various police departments. You'd find credentials and contact cards for plumbers, electricians, insurance agents, mechanics, health inspectors, private investigators, news reporters, funeral directors, and a few more exotic ones like professional bigfoot spotters, pre-Babylonian art professors, and clown hairdressers.
Don't ask.
Throughout Sam's life he'd been countless actors, directors, and band members, washed-up has-beens and obscure nobodies. He'd gone by Jimmy Page, Wedge Antilles, and Jerry Kaplan. Bachman, Berkowitz, Johnson, Jones, and, apparently, Wesson, or so Dean had told him. For about a week or so he was known as Keith, but he was trying to forget about that.
In other words, Sam went through personas like the Ghostfacers went through interns.
All that pretense, added to the fact that Sam's birth certificate was probably the only documented proof of his real name (if his father hadn't gone back and destroyed it for security purposes, that is), you might say that an identity crisis or two was more than justifiable.
But identity had never been a problem for Sam. Sure, he had clashed with his father over being a hunter. Had unknowingly played for the bad guys with devastating consequences. Had struggled to come to terms with a seemingly inescapable destiny. But through it all he'd always had a rock-solid basis for who he was: his brother. In a life that changed drastically day to day, Dean was a constant. He was always right at Sam's side, faking it till he made it right along with him. Even when Sam did lose himself, it was always Dean that brought him back. No matter what else happened—who else he pretended or tried to be—Sam would always be Dean's little brother.
And that was all the identity he needed.
Happy Halloween!