This question already has an answer here:
I have problem with one deb repository (specifically medibuntu.org) today - synaptic is not able to find repository indexes (and they are really not there - maybe due to yesterday's changes).
So I have the errors flying all over the place. I could just remove that repository - but I simply do not remember which packages came from it, so just removing it could screw my updates on some crucial package.
So, my question is - how do I find, which installed packages can not be found in any repository?