In my one-person crusade to educate the world about writing, fiction writing in particular, I commonly encounter the following scenario in advertisements seeking ghostwriters: "I'll tell you my idea, you write the book, and I'll give you a half share in the royalties."
Um... no. As I've stated ad nauseum, ideas are easy and plentiful. Developing them is hard work. It takes time, effort, and skill to produce something worth reading.
But how does you know whether an idea is a good idea? Ask Google and you'll get 129,000,000 results. That alone should tell you that this question occurs frequently.
Writer's Digest publishes a list of questions and actions to determine whether an idea can sustain development to novel length (minimum 50,000 words). The Balance also offers a list of qualifying questions that will help the writer determine whether an idea has the strength to endure a novel's length. Jane Friedman's website takes a somewhat different approach in emphasizing that if you're writing on a popular theme, agents and publishers won't want your manuscript. She distills the previous lists into three succinct questions: 1) Why make this? 2) Why make this now? 3) Who cares? She goes further to touch upon the question authors should not ask agents and publishers: What are you looking for? After bursting a writer's bubble, Damien Walter offers "six core qualities for a strong commercial novel." Those are high concept, larger than life characters, inspiring locations, close relationships, high stakes, and multiple points of view.
These qualifying questions and traits work well for those writers who write out their plots, develop outlines, and otherwise proceed through the story development process in a methodical, disciplined manner. Authors of this type are called "plotters."
Unfortunately, that probably won't work for "pantsers." Yep, you got it. I'm a pantser. I can work from an outline--someone else's outline. Often the writing process is as much one of discovery for me as it is for the characters and the reader. As Robin McKinley writes in her book Beauty, "Begin in the middle and work outwards. Don't be stuffy."
As an English teacher once said about good story ideas: "Drop an ordinary person into an extraordinary situation and let him or her resolve the conflict." That's as good advice as any, to my way of thinking.
So how do I tell whether an idea is good, whether it will support 50,000 or 100,000 words? The idea has to hold my attention. A powerful idea simmers in my brain, won't allow me to relax my mind, and practically forces me to either put fingertips to keyboard or temporarily drown the voices with the liberal consumption of wine. (Being that warm weather has arrived, wine may take a back seat to gin and tonic.) The upshot is that the story holds me in its grip. It's on my mind when I wake and when I go to bed.
The story must be expressed or my head will explode. I don't write because I have nothing better to do; I write because I must.
Ultimately, I think, the idea doesn't matter so much as the development and execution. Think about it: how many variations on Cinderella have you come across? It's a simple idea: poor, downtrodden woman meets wealthy, privileged man; they fall in love; a problem prevents them from pursuing a relationship; they resolve the conflict and live happily ever after. And, yet, the idea endures despite its repetition across genres and centuries.
Music follows the same general order. Some musicians and groups produce great music that endures, some one-song wonders and currently popular bands produce a hit or two that the public won't remember in ten, twenty, or thirty years. People today still listen to Mozart, the Eagles, and Bessie Smith. Something resonates with the music they produced that still captivates the listening public today.
So, back to the original question: Is the idea good? Only the writer can truly answer that and readers will validate that answer.