How to find out the version of jdk that is used to generate .class file ?

ANS-------------------->

Use javap tool for this as shown below 
>javap -verbose <classname>


deside the jdk version based on major version 

__________________________
major version      |   jdk                 |
++++++++++++++++++++++++
     52                 |  1.8                  |
++++++++++++++++++++++++
    51                 |   1.7                  |
++++++++++++++++++++++++
   50                 +   1.6                  |
++++++++++++++++++++++++