This question already has an answer here:
Clearly not every marvel movie is part of the MCU. Off the top of my head, there is:
Tobey Macguire Spiderman 1, 2, and 3
Andrew Garfield Spiderman 1 and 2
Fantastic Four and Rise of the Silver Surfer
New Fantastic Four
The bagillion XMen movies
Hulk (Not "The Incredible Hulk")
So if all of these Marvel movies aren't part of the MCU, why are the Iron Man, Thor, Avengers, Captain America and other series part of the MCU?
I can think of a couple reasons they might not count. For example, both Spiderman series and the original Hulk don't count because they have new versions of the same character. But this doesn't explain all of them.
Another reason I thought of is that a movie doesn't count as part of the MCU until it clearly establishes a continuity with the world of Iron Man and The Avengers. This would explain for example, Deadpool, XMen, and Fantastic Four. But this still doesn't explain a couple of things. For example, Ant-man had no shared continuity until Civil-War came out. And Guardians of the Galaxy has no continuity with the avengers, at least, that has been revealed yet (although I would not be surprised if this happens with Infinity-Wars). I still don't even know if Guardians of the Galaxy happens before or after The Avengers. And yet, both of these movies count as part of the MCU.
So how can you clearly determine whether or not any given movie is officially part of the Marvel Cinematic Universe?