How to make sure I'm not using any proprietary software after installation?
vrms
(V
irtual R
ichard M
. S
tallman) to the rescue:
sudo apt install vrms
vrms
and you'll get something like:
Non-free packages installed on computer-name
app-shortname1 Application long name 1
app-shortname2 Application long name 2
app-shortname3 Application long name 3
Contrib packages installed on computer-name
app-contrib1 Application Contrib Name 1
3 non-free packages, 0.4% of 2381 installed packages.
1 contrib packages, 0.1% of 2381 installed packages.
that will ensure you do / do not have any proprietary software installed.
Note: CPU firmware patches are non-free but absolutely needed to protect you from some processor flaws and their ilks, so don't go overboard! ;-)
If you're keen on having only Free Software (although you used the term Open Source) on your computer, you may want to consider installing Ubuntu with the "Free Software Only" option. You can find out more over at this question.