I wanted to know if 'uncouthed' is a word or not. I didn't find anything when I searched for it in dictionaries. All I got was the word 'uncouth'. However, when I searched it in Google Books, it showed a lot of books where this word is used.
So what's the best way/website to know if a word exists or not.
You don't. There isn't, like, an official list of official words. You just have to give it a sensible guess or look it up in a dictionary to see if it looks credible.
Most dictionaries record language, they don't tell you if a word is "official" or not, only that it has been used before often enough to be recorded. Words all start off being made up by someone and then get used often enough to be no longer considered made-up.
Additionally, there isn't really a solid definition of the word word in linguistics so it's all rather vague anyway.
English is quite flexible about what you can do with words. Most nouns can be verbed and there are various affixes that can make adjectives into verbs - I could say that I've de-stinkified myself if I had a really nice shower after being a bit stinky and I think most people would consider that "not a real word" but, really, if it works like a word then you could argue that it is a word that now exists.
If you want words that have an established history of being used, Wiktionary is as good as any other source, or any online dictionary really. Merriam-Webster is good. The Oxford English Dictionary has almost every word ever used as part of English, although even they accept that words that come and go very quickly will go unrecorded.
