I dont look for exactly where they break (that's usually debatable) but for discussion sake it's usually not hard to identify certain actions as in the first act (set up) second act (rising action) and third act (climax and resoltion).

In college I took my share of film classes, and there was this one dude, my age (early 20s in the late 90s) who always referred to "reels." He was not a projectionist or anything, but he insisted on referreing to parts of movies (that we always saw on DVD or Laserdisc) as taking part in this "reel" or that "reel.'

I hated that guy.