Here's the simplest answer to your question: if you at any time accepted the warning given in the preamble to all descriptions of how to enable Extras-devel or Extras-testing repository, and installed one or more apps from those repositories, you should know about it. And those are the only candidates if you've got unoptified packages installed. If in doubt, uninstall all the ones you installed from the testing and development repos.
Now, if your question is how do you know which packages are the offenders when looking through your root partition and finding large files, the dpkg -S is what you want. dpkg -S /opt/bounce/bin/bounce returns: bounce Which is the name of the package that file belongs to. Obviously this is a trivial case, but it is useful if you are browsing through your rootfs and find a big file, and want to know what package put it there. "Bounce" is not a filename anywhere on your system, which is why it didn't return anything. "bounce" would have returned a bunch of entries, most of them (not surprising) belonging to the bounce package. This post and the one it quoted may also be useful for you if you are trying to track down the culprits: http://talk.maemo.org/showpost.php?p...7&postcount=19 But the easiest way to be sure, as i said, is to remove anything in testing or development.
BTW, the solution "remove applications installed from Extras-devel or Extras-testing" does not work actually - most of them pulls HUGE libraries to root file space and that libraries are NOT automatically removed by application removal. You should target it specifically with red pill ON + careful research of packages dependencies OR use 'dpkg autoremove' from X-Terminal root shell... with some luck...