How to find what packages do not come from a repository?

Run Synaptic. In the lower left series of buttons, select “Status”. The list above these buttons may have an entry “Installed (local or obsolete)“. When selected, this will show you all packages that were either installed locally (eg. from a downloaded deb file) or were installed via a repository but aren’t listed any more. (There’s no real way for Synaptic to tell the difference, hence they’re grouped together.)

If you don’t have any local or obsolete packages, this entry will not be present, so don’t worry 🙂