As a general rule, you should make parallel any two (or three or more) things that are put forth in the same way, or could be thought of as being on the same level. I find it helpful to think of "X" and "Y" elements - if you can express the elements in terms of X and Y, you probably have a parallel structure.
It's helpful to memorize a lot of the common parallel indicators:
X and Y
X, Y, and Z
X or Y
either X or Y
neither X nor Y
X rather than Y
not X but Y
not only X but also Y
from X to Y
There are others, but these are the most common.
I don't think that it's necessarily helpful to have a check-list for SC, such as "subject-verb? check! pronouns? check!" This might get exhausting, and you might miss important meaning distinctions. I think the best thing to do is read for structure - where are the clauses? Are there any parallel or comparative constructions? Where are the modifiers? Then, as you're looking through the answer choices, read vertically to note any differences.
Ceilidh Erickson
EdM in Mind, Brain, and Education
Harvard Graduate School of Education
