"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.
Lot of 10pcs Brocade 57-1000012-01 8Gbps SWL 850nm SFP+ Optical Transceivers
$19.00
Cisco Nexus 48-Port 10G SFP+ Switch N9K-9396PX w/ 9K-M12PQ 12-Port 40G QSFP
$249.99
Cisco Meraki MA-SFP-1GB-SX 1000BASE-SX SFP Transceiver Module
$24.99
Cisco GLC-FE-100FX-RGD 100BASE-FX SFP Transceiver Module
$17.99
Genuine Cisco SFP-10G-SR V03 10GBASE-SR SFP+ Transceiver Module 10-2415-03
$8.00
Brand New Cisco GLC-LH-SMD 1000BASE-LX/LH SFP Module 1310nm 10km SMF LC
$13.89
Lot of 10 - HP 10Gb SR SFP+ Transceiver 455883-B21 455885-001 456096-001 850nm
$23.99
SFP-10G-SR Original Cisco 10GBASE-SR SFP+ V02 Multi mode Transceiver 10-2415-02
$5.00
LOT OF 20 Genuine Cisco SFP-10G-SR V03 10GBASE-SR SFP+ Transceiver Module
$89.00
SFP-10G-SR V03 Original CISCO 10-2415-03 850nm 10GBASE-SR SFP+ Multi mode Module
$6.80