How would one go about finding out if their oh so original idea for a story wasn't original at all?
As Haggis correctly mentions, most original ideas have already been used. There's not much new to add. The Bible(s) alone takes up the majority of literary themes.
If you're in the market for originality, it's already been done before, no matter what the story, at least a thousand times over, if not more. The good news is, you can change what's already been written to fit your own perspective, to energize age-old ideas and stories to fit your own brand of description.
That's what keeps readers entranced, hearing the same stories over and over but told from different points of views that sound fresh and entirely new each time.
My dad used to say, "There's nothing new under the sun, Son." And I'd think,
Maybe not, but if I can change it around a little and make it sound different,
or maybe even better, I'd be on to something worth writing about.