In contemporary literature, it is usually relatively easy to identify the protagonist (or alternating protagonists if we accept that there can even be such a thing as more than one protagonist—something I'm not inclined to dispute, though there are those who would and do).
But it gets more complicated if you look at 19th and early twentieth century literature, because there are many classic stories where the question of just who is the protagonist may be exceedingly blurred. For example, the Sherlock Holmes stories rarely present Holmes's viewpoint. The stories are generally told in Dr. Watson's viewpoint, as a first person narrator, and though he often takes some small part in the action, he certainly doesn't drive it. It's the opposite, really, because when he does take part in the resolution of the story, he does so on the instructions of Holmes, fulfilling some role assigned to him by the detective (and not always clear about what that role truly is), so he doesn't even drive his own actions regarding the mystery plot. It is Watson who tells the story (with one or two exceptions), Watson with whom the reader identifies and sympathizes (Holmes, by contrast, is far too cold to attract much in the way of sympathy) which for most readers most of the time would identify a character as the protagonist, and yet it is Holmes around whom the story revolves, Holmes who drives the plot, Holmes whom (I believe) most readers would identify as the protagonist. And certainly interest in Holmes and how he will solve the case is the reason why readers read the story in the first place. Which is just as Doyle plans it.
In (The) Strange Case of Dr. Jekyll and Mr Hyde, by Robert Louis Stevenson, except for in one letter near the end where Dr. Jekyll explains himself, the story is told from the viewpoint of Jekyll's friend Utterson, who observes some of the action, has other parts of the action described to him by other observers. In terms of investigating and solving the mystery of what it all means, of what is really happening, Utterson does drive the story and is generally considered the protagonist (when he is remembered at all, that is) and yet is so unimportant to the central story that most dramatic adaptations give him no role at all.
In Wuthering Heights, by Emily Brontë, neither Heathcliff nor Cathy appears as the viewpoint character (except brief fragments of writing that come to light), though they are certainly the main characters, the characters most active in and acted on by the events, the characters who drive those events, a role which nobody else plays exceptCatherine's young daughter later in the story The main viewpoint characters are Nelly Dean the housekeeper, who narrates most of the story to the new tenant of Thrushcross Grange, Mr. Lockwood, or Lockwood himself who observes later events. Nelly does play a minor role in the action of the main plot, but she certainly doesn't make things happen or effect the other characters (though she tries, bless her), and though the story would be incomplete without Lockwood's observations of scenes which Nelly never sees, I don't think anyone would really consider either of them the protagonists.
And then there are numerous Victorian gothic tales and ghost stories where there is a framing story involving a character who narrates the story, who serves as a viewpoint for much of it, who may take some role in the events, but again is an observer rather than an engine of the plot. For example, "The Old Nurse's Tale" by Elizabeth Gaskell.
All of the above (and many others like them) undoubtedly represent successful storytelling (and I speak of being artistically successful, not just of their longevity and popularity), so depending on the stories we choose to tell, recognizing for ourselves the protagonist(s) and portraying them in such a way that critics and other readers may clearly know who they are may not be quite so vitally important as we tend to think.
If not rigidly following (modern) guidelines about what a protagonist is and does was good enough for the likes of Doyle, Stevenson, Brontë , and Gaskell, then it should be good enough for us—if, that is, we are skilled enough to pull off our stories as well as they did theirs.