The compounds having same molecular formula but different structural formula are called Isomers. To identify Isomers is to look upon that compound or element which is also an isomer of that substance and count the no. element present in it & if it becomes same than look upon its structure & if the structure is different then it is its isomer & this property in organic chemistry is called isomerism
						

							
Hi ajay , many of intemesiate find it difficult in isomerism in the organic part of chemistry
  Isomers are the structures having same molecular formula but dfrnt physical and chemical properties
 it is primarily classified as structural isomerism and stereoisomerism which is further deeply classified as follows
                                                                   Isomerism
                                     ________________|_______________
                                     |                                                              |
                             structural                                                   stereo
                               isomers                                                    isomers                                                                     
                         *chain isomers                                   ________|___________           
                          *positional isomers                           |                                  |                                       
                         *functional isomers                   configurational             conformational
                          *metamers                                   isomers                        isomers
                            *ring chain isomers      _________|_______
                          *tautomers                    |                                |
                                                geometrical isomers           optical isomers
 
However the most of all is geometrical isomers 
  They can only be known through a scientific thinking process and by presenting the given compound in a systematic manner,and when you learn there vl really b no need to memorise anything and the things go right during exam,there is nothing like shortcut formulae for this phenomenon.
      for example if u are gven to draw isomers forc6h12  then first draw 6 carbon chain then 5 carbon chain then 4 and so on in the process do not bother about no of hydrogens coz they really get balanced out if u r really doubtfull u can verify them at last.for better results name the compounds u get through iupac rules  so that u can eliminate if repeted.
                            Refer O.PTandon for further detailation if required 
                               enjoy isomerising compounds for then hope it was useful