If you have absolutely  no idea what the answer is........and if u hav some idea about the concept.....then the following can useful........
1.If one the answer is substantially longer,it could be the right answer?
2.If there are two exact opposites the answer could be one of the two.
3.Statements that are ardent and use definitive words, such as "always ",
 "all","must","every time","none","never", are usually false.
4.Statements that have language, such as "usually","most often","likely","probably","could","might","should", and "rarely",are always true
5.If two answers sound alike,choose one of them.
6.If two numbers differ only by decimal ponit(and others are not close ) choose one of them..ex- 2.3,6,1.5,15 we can either choose 1.5 or 15.
7.When u don't know the right answer , look for the wrong one to eliminate options.
8.If u are supposed to read a passage and answer questions about it, read the question  first.That will tell you what  you're looking for...
NOTE-always remember a guess is a guess.
          don't always try it...apply it where ever necessary
SOURCE----- SURE-FIRE TIPS TO CRACK EXAMS