Trivia: Apocalypse's followers are referred to as The Four Horsemen in the comic, and there is also a brief mention of it in the movie (when Charles and Alex go to see Moira). The song playing when apocalypse goes to recruit Angel is "The Four Horsemen", by Metallica.
Trivia: The program playing on the TV at Ororo Munroe / Storm's home which Apocalypse watches is the 1967 Star Trek episode "Who Mourns for Adonais?" which also features the return of a godlike being.
Trivia: With this film, Bryan Singer broke the record and became the first director to have made four movies in a single superhero franchise. Singer directed "X-Men", "X2", "X-Men: Days of Future Past" and "X-Men: Apocalypse." This record was eventually matched three years later by the Russo Brothers, who have made four films in the Marvel Cinematic Universe to date.
Trivia: Kodi Smit-McPhee, who plays Nightcrawler, was originally cast to play young Logan in X-Men Origins: Wolverine before dropping out to star in The Road instead.
Trivia: Zendaya was considered for the role of Storm, but ultimately not chosen. The following year she would appear in another Marvel film - "Spider-Man: Homecoming."
Trivia: The original title of the film was "X-Men: Age of Apocalypse." This was changed because coincidentally, around the same time, three other films with "Age of..." in their titles had come out: "Avengers: Age of Ultron," "Transformers: Age of Extinction" and "The Age of Adeline."
Trivia: As well as making his usual cameo appearance, Stan Lee appears with his wife Joan for his cameo in this film.
Trivia: At the end of the movie, when Erik and Charles are saying their goodbyes, Erik asks Charles "Does it ever keep you up at night, the feeling one day they will come for you and your children", to which Charles replies "Yes it does, and I feel a great swell of pity for the poor soul who comes looking for trouble at my school." They have the same conversation, almost word-for-word, at the end of the original X-Men.
Trivia: SPOILER: After Jean Grey fights off Apocalypse, the fire that surrounds her takes on the brief shape of a fiery Phoenix, an allusion to an X-Men storyline in the comics.
Trivia: When they leave the cinema after watching Return of the Jedi, Jean Grey quips that "at least we can all agree the third one is the weakest" or words to that effect, a sly dig at X-Men: The Last Stand which is considered the weakest of the original trilogy and not directed by Bryan Singer.
Trivia: Michael Fassbender is only ten years older than his on-screen son Evan Peters.
Trivia: Jubilee, Nightcrawler, Jean and Scott are seen leaving the theatre after watching Star Wars: Episode VI - Return of the Jedi (1983). Oscar Isaac, who plays villain Apocalypse, starred in Star Wars: Episode VII - The Force Awakens (2015).
Answer: As shown in Days of Future past, Trask was able to identify Magneto and Mystique, as well as their mutant powers. Trask could have found Charles Xavier at any given time, or Stryker possibly researched him. Stryker did work for Trask after all.