In eclipse if you want to know what jar a class is in ctrl-shift-T will show you the full path to the jar in your search results.
Intellij makes you click "include non-project classes" and then navigate to the class, then hover over the classes tab to see the jar. And if you are using Jad this doesn't work and are left with no solution that I know of.
Is there a better way?
If not, I'm going to file a feature request.