When Dean is seen working on the car in the beginning. He is wearing a short sleeve shirt and the Mark of Cain is nowhere to be seen on his arm.
In the opening scene Marie refers to Sam and Dean as "the Winchesters". In season 4 episode 18 when Sam and Dean meet Edlund, Edlund says he never used their last name in the books.
However, Becky Rosen (from "The Real Ghostbusters" in season 5 and "Season Seven, Time for a Wedding!" in season 7 knew the brothers' last name and was known to post information about them on Supernatural fan websites using the screen name "beckywinchester176" (as stated by Charlie in the season 9 episode "Slumber Party." It is not inconceivable that Becky revealed Sam and Dean's last name in one of her posts.
However, Becky Rosen (from "The Real Ghostbusters" in season 5 and "Season Seven, Time for a Wedding!" in season 7 knew the brothers' last name and was known to post information about them on Supernatural fan websites using the screen name "beckywinchester176" (as stated by Charlie in the season 9 episode "Slumber Party." It is not inconceivable that Becky revealed Sam and Dean's last name in one of her posts.
In the beginning, the writer is writing the Supernatural pilot episode script using Windows Vista or higher (Aero cursor first showed up in Vista). Vista was released in 2007 and the pilot was aired in 2005.