Verify java version when running jar file

I have a problem that seems simple, but has been stumping me for some
time.  I have an executable jar file that I have distributed out to
other people.  It relies on Java 1.4.2, but some of the customers have
1.3.  Is there anything that I can do in the Manifest or Java code to
catch this problem right away, so that the application can fail
gracefully?  Currently, it just ends up at a NoClassDefFound.

I would appreciate any suggestions.

Thanks!