You know, there was one "revelation" that happened in Civil War that really comes out of left-field if you didn't watch Winter Soldier and really pay attention...
That Bucky was involved with the death of Tony Starks parents. When they watch the tape and Tony asks if Steve "knew", Steve says "I didn't know it was him." before Tony repeats himself with Steve admitting "Yes". For those that may not remember, in Winter Soldier, when Steve and Natasha head to the old SHIELD bunker and find the Zola computer, he stalls by telling them the story of HYDRA growing within SHIELD, and at one point he shows them a picture of Howard Stark's death and the line "accidents... will happen".
That single little blurb is the only reason Steve knew, since before that, Howard Stark's death was chalked up to a car accident since Iron Man 1. That one moment was the only lead up to Steve knowing about the murder, which is why when he admits to Tony that he "knew" about his parents it feels rather out of nowhere in the context of the movie.