The details are given in Thiam Teck’s Blog.
Basically, the command
javap -verbose ClassName
is given and the extensive dump is mined for the major version and minor version of java used. The cited blog gives a table of major/minor pairs that go from 1.0 through 1.6.
CA FE BA BE 00 03 00 2D
The first four bytes
CA FE BA BE are a signature for a .CLASS file, while
the next two bytes are the minor version (3) and the major version (2D or 45
base 10). The cited article (and the JVM documentation) will match this up to
JVM 1.1 for the example file. Version 1.5 of the JVM runs this class file with
no problem. Real’s How To also gives a sample program for those who have a