When I run pacman -Qe I get a list of explicitly installed packages.
However on Manjaro, some of these packages were there already when I first installed manjaro, and others were added explicitly by me. How do I distinguish between the two?
I’d like to get a list of packages which I installed myself, and were not included in the distribution and are not dependencies of other packages.
You can check the pacman log at /var/log/pacman.log to see what packages have been installed since you installed the system.
But explicitly installed, also means the packages that get picked by the Manjaro Team on the ISO.
To see what packages that might be, you should check the iso-profiles repo at Projects · Profiles & Settings / iso-profiles · GitLab and find the edition you used.