To know whether you're running Java 5, 6 or 7, firstly type java -version.
There will be a line in your output that looks similar to this: java version "1.7.0_55"
Then you use this table for converting the 'jargon' result to the version number.
1.7.0_55 is Java 7
1.6.0_75 is Java 6
1.5.0_65 is Java 5