In theory, it doesn’t seem hard to establish who your main character is. After all, your main character is the main character because the reader spends the most time with him. If the story is in first person, he’s even the narrator, but at the very least, you probably get his thoughts and perspective, something you wouldn’t from other characters.
Or maybe your main character is the main character because all the stuff happens to her. She’s not just some random waitress who shows up once in the middle scene and has a few witty lines. She is the key to the whole entire plot!
Sorry, but that’s just not enough. I’ve read stories where the main character is not the main character. Sometimes, there’s a gaping hole where the main character should be. Other times, there’s a sidekick who’s practically demanding to be put in the spotlight. Either way, the story won’t work like it’s supposed to without a strong protagonist, the right protagonist. Here’s a checklist to make sure you have one—we’ll call him/her the “MC.”