Don't rely on suffixes or prefixes. This is why:
DE (down), EX (out) and RE (back)
These aren't just oddball exceptions to the rule. Words often stray a long way from their suffixes, prefixes and roots. If you tried to rely on suffixes and prefixes here, you'd waste about one minute to find out that decry means to denounce (down-cry, sorta denounce). Unfortunately, with that wasted minute you also realized that you can't figure out a thing about the other three (or, more likely, you discerned the wrong meaning). Don't waste time memorizing suffixes and prefixes, and more importantly, don't waste time on your test trying to figure out the meaning of a word based on them. Either you know the word or you don't.