"apt-mark showmanual" will show you all packages that you have manually installed
"apt-mark showauto" will show you all packages that are automatically installed

"apt-mark showinstall" will show you all the packages that are installed


How is this useful? It tells you what packages are installed in your system.

So you can go "apt-mark showinstall | grep somelibrary" to see if you have a particular library installed that's a dependency.