I know I can check the currently installed non-free software with the following command (assuming aptitude is FOSS):

aptitude search '?installed (?section(restricted) | ?section(multiverse))'

And I can remove the "restricted" and "multiverse" sources (and not add any PPA's) to keep that list empty. I'm assuming from this point, whenever I apt install something , that something is either in the allowed sources or it is not found.

Is this enough? Does this make sure all software I'm running (in Ubuntu) is open-source?