When you want to see with which jdk version a java class has been compiled you can run the following command
javap -verbose MyClassName
Then you see in the beginning of the output something like this
minor version: 0
major version: 49
Constant pool:
You want the major version from the results. According to this number and the following matrix you can determine the jdk version
Java 1.2 uses major version 46
Java 1.3 uses major version 47
Java 1.4 uses major version 48
Java 5 uses major version 49
Java 6 uses major version 50