I had a simple technique. I dedicated the first few days of my preparation to familiarise myself with both the syllabus and the previous question papers. This made sure that I was thorough with what needed to be studied and what need not.
After this, I simply started out with the standard books that were recommended for the subject. In most of the cases I knew whether a particular portion needed to be studied or not.
This was mostly applicable to dynamic portions like current affairs, and the optional papers. For static subjects, I read the complete book without fail. For example, in Laxmikanth, there is pretty much nothing that you can skip. You simply go by the logic that the book has been prepared with CSE in mind and hence would not contain anything out of syllabus.