"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.
Dell SNPTP9W1C/16G 16 GB 2666MHz (PC4-2666) Memory Ram
$34.95
A-Tech 8GB DDR3 1600 PC3-12800 Laptop SODIMM 204-Pin Memory RAM PC3L DDR3L 1x 8G
$13.99
NEW OLOy 16GB 2x8GB KIT DDR4-3000 PC4-24000 Desktop Gaming RAM MD4U0830160BB1ST
$19.99
Micron 8GB DDR4 1Rx16 PC4-3200AA Laptop RAM Memory MTA4ATF1G64HZ-3G2E2
$10.00
Lot of 2 4GB DDR4 Memory PC4-3200 SODIMM Laptop RAM 3200MHz
$9.99
Crucial DDR3L 16GB 1600 2x 8GB PC3-12800 Laptop SODIMM Memory RAM PC3 16G DDR3
$22.45
Team T-FORCE VULCAN Z 32GB (2 x 16GB) PC RAM DDR4 3200 (PC4 25600) Memory
$54.99
8GB PC3L-12800S 1600MHz SODIMM DDR3 RAM | Grade A
$12.00
HyperX FURY DDR4 8GB 16GB 32GB 3200MHz PC4-25600 Desktop RAM Memory DIMM 288pins
$71.93
256GB Kit (8 x 32GB) PC4-17000 LRDIMM DELL POWEREDGE R730xd R730 R630 Memory RAM
$255.92