How can I differentiate countable noun and uncountable noun?
As a native speaker, if you've just heard a new meme word, do you recogize that it is countable or uncountable?

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You don't automatically know just from hearing it unless it's used with a number or a/an, in which case it's countable. A lot of the time, you just have to guess from context and the way it's used, and some new words can be used both as a countable and uncountable noun because people disagree.

If I heard the sentence "I bought a new sindel" (not a real word) then I know it's countable, but if I heard the sentence "Have you received my helt yet?" (also not a real word), I can't tell whether it's countable or not because both countable and uncountable nouns fit in this sentence.