in Unix : javap -verbose MyClass | grep "major"

 

in Windows : javap -verbose MyClass | findstr "major"

D:\src\com\test\service\>javap -verbose WorkTest | findstr "major"
Warning: Binary file WorkflowMessage contains com.test.service.WorkTest
  major version: 51

* Major version 참고

51 : jdk1.7

52 : jdk1.8