Such an interesting question! Such thought-provoking remarks in the comments beneath the question!
Such comments make me wonder, what is the best way to answer such a query? Maybe a few things need to be addressed first, such as:
- What is the meaning of such?
- How is it used in conversation and such?
- What is grammatical, and what sounds natural?
A good online dictionary, such as Wordnik, might help. There, we can find such a great number of meanings and examples, that it will become readily apparent that such a question is not so easily answered. English can be such a hard thing to fully grasp, particularly when dealing with such words – words that can be used in such a myriad of ways! After all, many words, though but one word, can be used as different parts of speech: adjectives, adverbs, pronouns and such; such is one such word.
Really, Ivan, you should go visit such a website, and behold the diverse ways such a simple word can be used. Such a curious man is bound to learn something from such a great smattering of examples!
which is to say, there's no EASY way to tell when to use "such", and when to use "such a" - the word is simply too flexible to nail down in such a fashion...