1. Use the split-resplit method for starters manhattan-gmat-split-and-re-split-method-106198.html
. This is what is meant as the vertical scan by the above poster.
2. Try to identify keywords which can help spot possible errors. Some examples (a) it/they/them etc will signify pronoun error (b) and will signify parallelism (c) than, more, greater will signify comparison etc.
3. Over and above using grammar rules, try to rely on logic i.e. what the sentence is trying to convey. Many a times people lose track of this in pursuit of grammatical flaws.
4. Preposition signify an idiom is lurking around. Ensure that you mug up any decent idiom book (part of any preparatory guide) and remember them well.
5. If there are long modifier then ensure they modify the current noun or verb. Once done, try striking them off and ensure you are not left with run-on sentences or fragments.
These are just some tips you need to remember when trying spot a sentence. As you practice more I am sure you will come up with your own list.