You can't ever be entirely sure if an idea wasn't thought of before. But, if you care to demonstrate originality, you can try an extensive literature review to see if anyone else has thought of the same idea. After that, the best you can say is that you haven't seen anyone else with the same idea.
Personally, I don't think being the first person to have an idea is worth much. It depends entirely on what you do with it. I tend to do detailed literature reviews because they help me generate ideas, not because they help me verify that my ideas are original.
extensive literature review
I'm a random person on the internet; what sort of sources would be used in such a review?
If it's worth saying, but not worth its own post (even in Discussion), then it goes here.
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