Really it's variable, because you can assign which release you want to use as long as the release version you use is earlier then the one you are compiling with. So how to find what the latest version javac is using to compile?
run
javac -source 1.5
If it is using 1.5 javac you get message "no source files"
If it is not using 1.5 you get message "invalid source release"
Run the same
test using 1.4, 1.3 ... on and on..
Cheers