JarCheck

Check a Java jar to make sure all the javac -target versions of
the class files are what you expect.

Java application.
Requires Java version 1.5 or later.

All Java source code is included.

to use:

java -jar jarcheck.jar   jartotocheck.jar 1.1 1.4

where jartocheck.jar is the name of jar to check.
It will check all the class files in it.

In this example:
1.1 is the lowest acceptable target version number.
1.4 is the highest acceptable target version number.

Currently, only version numbers 1.0, 1.1, 1.2, 1.3, 1.4, 1.5
and 1.6 are supported. The program does not currently test
minor version numbers.

Why would you use this utility? To make sure that a jar
intended to run under an old 1.1 JVM contains no classes
accidentally compiled with javac -target 1.5.  Passing this
test does not guarantee the jar will work on an old JVM. The
jar still might use methods not bundled with old JVMs. This
should catch however, classes normally compiled with target
1.1 that were accidentally compiled with 1.5 as a side
effect of compiling some 1.5 class.

There is a similar program called JarLook available at
http://mindprod.com/products1.html#JARLOOK