Distinguishing a count noun (or countable noun) from an uncountable noun - This is a bit tricky, because a noun can be both or in some cases one or the other. For nouns that can be both (like truth) it depends on the use in the sentence. "There is no truth in what he said" (uncountable) versus "The fundamental truths about human nature" (countable). In the first sentence the word truth is referring the state of being (or quality) of 'what he said'. In the second, it is a reference to ideas that are true. Another example of both depending on use is milk, coffee, drinks of any sort (even the word drink). 'Would you like something to drink?" (uncountable - as it refers to drinks in general; you can change the word drink with milk or coffee or tea or juice, etc) versus "I order a drink" (countable as it refers directly to a singular drink that I ordered). Certain uncountable nouns are mass nouns - rain, sand, snow, flour, etc. Where unless stated otherwise it is just a mass "Look rain" refers to a plural (a large mass of rain) versus "look a rain drop" refers to a specified drop of rain. The mass ones are about memorization sadly. And there some tricky ones like data. A lot English speakers refer to data in the singular, but the word data is plural. And again many English speakers when they know this, will say something like "a data point" because they are unaware that datum is the singular form of the word. As for your last question, could you please clarify what you mean?

Are you asking 'how do I tell if a noun is countable or not?'? If so, think about it literally: can you count the noun or not? E.g. Apples (countable): You can pick up one apple, or two apples, or three apples. Therefore, a countable noun Water (non-countable): Can you pick up one water, or two waters? No. Can you count water? No. Therefore non-countable. Make sense? (quick tip: some people get confused over something like this. They will say 'how is coffee a non-countable noun? I can count coffee: I had three cups of coffee yesterday.' Here, of course, you are counting cups, not coffee).

You should memorize them pretty much, i did the same