During the take-off of the plane, the Boeing 737 is shown twice from behind - with the distinctive engines that are flattened at the bottom. When the plane is in the air filmed from the front, it's another B737 with the older, cigar-shaped engines.
The "Cold Drinks" vending machine seen in the CalTech cafeteria is the same one seen in almost every episode of The Big Bang Theory (2007). This indicates that the vending machine has been in the cafeteria since the early 1990s. EDIT: The "Cold Drinks" vending machine is a commonly used prop that has appeared in many television series.
Sheldon hears a computer-generated voice on TV and it reminds him of Steven Hawking, but Hawking did not have the computer-aided voice in the early '90s.
As Sheldon's computer is booting up, the line "DOSKEY installed" is visible on the monitor. The DOSKEY utility was first used in MS-DOS 5, which was released June 1991. The time setting of this episode is the beginning of 1991, before the release of MS-DOS 5.