The Jax project from alphaworks.ibm.com does a _really_ good job of figuring this out for you. it also optimizes, inlines, obfuscates, transforms and other useful functions. there is currently a commerical version available for 1k or so, though as far as i know the old free version is still around.

warning though: it's _very_ fiddly and tends to break things like serialization.

I'm not sure if there's a tool out there that could do what you want, but here's a possible solution to home-brew:

Write a program that opens up each class file in a JAR and reads the "Class" references in each class's constant pool. Then, you've got a list of classes and their dependencies.

Now, you can search through any JAR file to see if any of those dependent classes are present in the JAR file.

You can probably optimize this by ignoring any class references that start with 'java/', since it is likely that most files will have dozens of references to those classes, and they won't be of interest to you.

(I might be persuaded to write such a tool, if there was significant interest).

-chris