A good idea is one that inspires you. It energises you. It drives you to get to The End, whether that ending is 1000 words away, or 100,000 words away. If your motivation dries up half way through and you never complete the story you were writing, then the idea wasn't any good to begin with.
Because I teach creative writing, a considerable part of my life is spent listening to other people's ideas. There have been a few occasions when I've thought 'oh, now that's a good idea', but it doesn't happen often. That's not to say that I don't hear good stories; that does happen often. But the idea - the germ behind the story - is usually fairly straightforward. Similarly, I don't think I've heard a single story where I thought the idea behind it was 'bad'.
There isn't some wonderful Idea-o-meter that tells you if your idea is good or bad. It's what the author does to the story that makes it work, or not work. You just have to write the story out and see what happens. If you get to The End it was a good idea after all.