"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.
Seagate Exos 7E10 ST2000NM000B 2TB 7200RPM SATA 6.0Gb/s 3.5" Internal Hard Drive
$29.99
Seagate 10TB 7200RPM 12Gbps 3.5in SAS Hard Drive ST10000NM096
$79.99
Toshiba 4TB, SATA 3, 3.5'' Internal Hard Drive (MG03ACA400)
$48.00
WF12F DELL 1TB 7.2K 6GBPS SATA 2.5'' HDD HARD DRIVE ST91000640NS 0WF12F
$25.00
HGST Ultrastar DC HC520 12TB SATA 6Gb 256MB 3.5" Enterprise HDD- HUH721212ALE601
$79.99
Seagate ST12000NM0127 12TB 256MB 7200RPM 3.5" SATA 6.0Gb/s Enterprise Hard Drive
$93.88
Seagate ST12000NM0127 12TB SATA 6Gb/s 256MB 7200RPM 3.5" Enterprise Hard Drive
$99.99
HGST Ultrastar HE10 HUH721010ALE600 10TB SATA 6Gb/s 7200RPM 3.5" Enterprise HDD
$69.99
Western Digital WD10JUCT 1TB 2.5" SATA 3Gb/s 5400 RPM 16MB Laptop Hard Drive
$17.98
HGST 0F27352 10TB 3.5" SAS 7200 RPM 12Gb/s Hard Drive 100% #73 v
$65.00