This may be a bit weird but do you do this? We live in a world where it’s very easy. Just Google ‘em.
Here are the test results for a couple of mine – both of them deriving from my rage at Brexit, so apologies in advance for the anger.
It was the doddering old and the uneducated that voted us out of Europe (really – demographics of the vote are well established). That is to say, it was the poor. All of my life I have voted Labour* - that might see me labelled as a socialist (the word doesn’t have the same connotations/stigma in the UK as in the US - for the avoidance of doubt, very roughly maps to Democrat). But now I have taken to calling myself a Two Nation Socialist.** Basically still a socialist, but screw the poor – they got us into this mess, and now they can suffer for it.
So how original a concept is Two Nation Socialist? Googled in quotes gets you* two* hits (!) one of which looks very suspect (the Des Moines Register - really??). So: a pretty original thought.
Still fuming: I look back to Britain in the old days (pre-EU), a grim grey place of mass poverty, cultural isolation and ignorance, and awful food. You just know we’re heading back there. Decades of exposure to Europe basically worked miracles, but miracles can be undone. Consider: for all those years of chanting the mantra Are We Not Men?, you can still forget how to drink from a cup, and go back to walking on all fours.
You got it – an allegory for Brexit: welcome to the Island of Doctor Moreau, where the beasts are reverting.
But an original thought? Test Results: Googling <”Doctor Moreau” Brexit> gets you 9290 hits.* Not original*. (Admittedly, a lot of these hits look like false positives; but there were also additional combinations I could have googled – with “Dr Moreau” for a start.)
So I’d like to know your original thoughts; and I invite you to test them for originality and submit the results of testing.
Or is it just me?
j
Anyone who writes a paper knows that the first section is a review of the literature, where you show that all other papers on the topic are not like yours, and not as good anyway. Neglect this and some reviewer will recommend rejecting your paper for lack of originality - especially because the already published similar paper may be his or hers.
So, nothing new about this at all. It was a lot harder back when I did my dissertation where you had to go to an actual library and look through actual books and journals.
So, your thought isn’t original. 
I’ve done this, oh, maybe a double handful of times, long ago. I quickly came to the conclusion that how ever original a thought is to me, my thought isn’t going to be original to lots of anybody else. I don’t do it anymore.
I was once involved in evaluating new algorithms, e.g. for patent applications. A common objection was “Somebody may have thought of that already.” My reaction was often “I hope so! If we’re first to think of it, that may be because it’s a bad idea!”
Patent awards depend on “novelty” and “non-obviousness” but paradoxically these tests sometimes favor the obvious! Some ideas are so obvious, no one bothers to mention them in papers … so there’s no published “prior art” and the ideas can be represented as “novel”!
Here are a few examples of original thoughts:
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I think it was Voyager who came up with “Escher Curve” to describe the version of the Laffer Curve favored by some right-wing thinkers: Tax Revenue is always inversely monotone over Tax rate so tax revenue is maximized when tax rate is zero!
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Right here at SDMB, I invented the term “bloated amygdala” to describe right-wing cognition. (This is based on brain physiology.) Googling I now see a few references to that term — are they copying me? 
But I Googled years ago and found that “bloated amygdala” was not novel: it occurred in a comment in a Wonkette blog, but the comment seems to be deleted now.
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I started a thread about Ben’s novel method to improve disk controller performance. I wondered whether anyone could re-discover that method with hints. Some posters assured me that disk performance was so thoroughly studied that there could be nothing new/interesting under the sun. Yet only one re-discovered Ben’s method — Hi, Dr. S ! — and disk controllers weren’t one of his specialties.
If you have a quite original thought, the problem you’re going to have in researching whether or not it’s entirely original is that you probably had to generate a vocabulary to express your thought — either inventing some new terms or using some existing terms in ways that depart from their ordinary usage. And a hypothetical person somewhere else coming up with the same idea won’t be likely to have expressed it in quite the same way.
Of course the language in which you express an idea could be thought of as part of the “original thought” formulation in the first place, (see septimus’s post about “bloated amygdala” directly above), but if the brillant notion were a patent-worthy invention that would do something that’s never really been done before, for example, then most of us would think that naming the process and the product differently doesn’t really mean it isn’t the same gizmo.
I’ve done it a fair bit and sometimes my ideas turned out to be original. The Bacon Easter Egg, for example, is my invention.