How do I know if my main character is... "main" enough?

I’ve written stories where the main character was mostly there to observe two characters that were the driving force of the story. I didn’t think it was appropriate for the readers to get inside the head of either of them because I wanted their motives to be less than obvious. I think it worked.

There are many books – but I can’t think of any off the top of my head. They are more narrators than main characters.

It’s certainly been done a lot. Consider The Great Gatsby. Consider David Copperfield. Consider Sherlock Holmes. Having a character other than the main character act as narrator is a good device for a lot of reasons, not least of which is it that can be awkward for a character to praise herself and describe her own heroic actions.

In that case your narrator is not your protagonist.

If it’s a first-person narrative from the POV of a minor or secondary character, there just has to be a reason for that character to be privy to all the protagonist’s actions and other main story elements.

It should still work, there are some where the narrative is done by someone who is not even part of the story.

I was going to mention The Great Gatsby, but I see @Jeruba beat me to it. I think it can be a really good literary tool for the reasons she mentioned. If you’re going to have a narrator who is a character rather than third person narration, I quite like it when the narrator isn’t as important a character as some others. It makes me feel more like I’m in the story – like I’m there and watching it unfold. It’s harder for me to get into the head of a great hero, for example.

I’m working on a short story myself, in which the narrator is actually kind of despicable, and the protagonist is another character she spends her time with. Over the course of the story, the protagonist will transform the narrator quite a bit.

Watch (or better yet, read) “Kick-ass”. It is set up pretty much that way from the moment when hit-girl comes on the scene. See if you like how it feels.

I just thought of an example, myself, Shawshank Redemption. Of course, I’ve seen AND read Kick-Ass. These are all good points. Thanks for the advice, everyone. I went back through my story (which is better than I remember) and tweaked it.
It’s a graphic novel, and all the narration boxes are told from Ruth’s point of view, but the story is about Kate.
I decided the boxes were all taken from Ruth’s diary (not dated, just excerpts).
At the end, when Ruth finds out Kate is a superheroine, Kate says something like, “Come on, I’ll explain everything.” Which makes up for times when I show things outside of Ruth’s ken.

Your main character doesn’t have to be the central character of what’s going on in the story. The main character is simply the perspective the author wishes to illustrate for the readers. This isn’t a book, but take Final Fantasy X for example. Tidus isn’t really the main character in the sense that he’s a summoner or someone of great importance. He’s just tagging along with the other characters. We just wish to see how his story ends.