The 'dummy it' is only used in certain, very specific situations. If you learn to recognize those situations, you're good.
On the GMAT, the most common case where you'll see a 'dummy it' is in sentences like this:
It is surprising that unemployment has decreased.
It was considered unusual for her to wake up before noon.
Based on the evidence available, it could be claimed that the trial had been unsuccessful.
Notice what those sentences have in common:
(dummy it) ('be' verb) (adjective or another verb) (rest of the sentence)
So, if you see something like this:
It is good that I won.
That fits the pattern, and you don't need to look for an antecedent for 'it'.
But if you see this:
'It is nice.'
The rest of the sentence is missing, so you'd need an antecedent.
Or if you see this:
'It surprised me.'
The 'be verb' is missing, so you need an antecedent.