On GMAT, if I am correct then, 'which' almost always refers to the noun it immediately follows.

For Ex:
We finally chose the coffee table towards the back of the store, which we thought would complement our living room furniture.

The example above contains wrong usage of 'which' because by the rule('which' always refers to the noun it immediately follows) 'which' in this sentence refers to 'the store' . But its not the store that complements the living room furniture, its the coffee table. So the on GMAT the above sentence would be considered wrong.

Correct sentence would be "We finally chose the coffee table towards the back of the store, because we thought it would complement our living room furniture."

Notice that we have replaced the pronoun 'which' by 'it'. 'It' in the above sentence correctly refers to 'the coffee table'.

'Which' only refers to nouns, not to phrases/clauses.

For Ex:
The storm lasted all weekend long, which made us feel depressed.

The above example also has wrong usage of 'which', because here which is trying to refer the whole phrase "The storm lasted all weekend long". This is not allowed on GMAT.

Correct sentence would be "Because the storm lasted all weekend long, we felt depressed."

Usually on GMAT, whenever you see 'which' used as a pronoun, just make sure it correctly refers to a noun and not to phrases/clauses.

Hope that helps.