The trivia items below may give away important plot points.
In this episode, Walter tells Peter that Olivia would be good for him because she can "see right through you." This could be seen as foreshadowing for the next episode, in which Olivia regains her ability to perceive things from the alternate universe through vision. She is able to "see" through Peter, making her aware that he is from the other universe.
While Walter is telling Peter about Walter's father's betrayal of the Nazis, Peter says that his grandfather couldn't have worked for the Nazis in the first place, since (Peter thinks) his grandfather left Germany in 1933, which Peter says was "long before the Nazis came to power." In fact, Adolf Hitler was sworn in as Chancellor on January 30, 1933, the date that is marked by most historians as the start of the "Machtergreifung"- the Nazis' seizure of power - which became complete with the Reichstag fire on February 27, 1933. This is another clue that Peter is from the alternate universe.