Keep your eye on the refrigerator. Starting with the 56th episode, a Superman magnet is placed on the fridge so that it is visible from all kitchen angles.
Check out the stereo rack. A superman statue sits beside the stereo from episode 81 on. The superman statue is visible in most apartment angles.
Listen closely for verbal references. Seinfeld uses terms like "Kryptonite," "Lex Luthor," the "Fortress of Solitude" and "The Bizarros." Pay close attention when Seinfeld discusses someone's weaknesses, his enemies, ex-girlfriends' apartments and a naked Lois Lane drawing hobby.
Check out the duds. The clothes he wears in the first season are often blue, yellow and red. These are more subtle references to the Man of Steel.
Backgrounds can hold clues too. Keep your eyes open for theater marquees with a Superman reference. Though most references are found in the apartment, some are on the street.
Conversation is where it's at. Many of the superman references occur when George, Elaine, Kramer and Jerry are talking. There is also talk of Jimmy Olsen, Aquaman, Elastic Man, Plastic Man and, of course, Lois Lane.
Visit the web, especially the Seinology website for a complete episode by episode break down of Superman and other comic book hero references. You may be surprised at just how many there are.