"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 PowerEdge R630 8SFF Server 2x E5-2680v4 2.4GHz =28 Cores 128GB H730 4xRJ45
$339.00
Dell PowerEdge R630 Server 2x E5-2680 V4 = 28 Cores S130 32GB RAM NEW 480GB SSD
$256.99
Dell Poweredge R630 Server 2x E5-2620 V4 =16 Cores | S130 | 32GB RAM | 2x trays
$171.99
DELL PowerEdge R730 16SFF Server 2x E5-2680v4 2.4GHz =28 Cores 64GB H730 4xRJ45
$317.00
Dell PowerEdge R630 Server 2x E5-2640v3 2.60Ghz 16-Core 64GB H330
$182.65
HP ProLiant DL360 G9 Server 2x E5-2690v3 2.60Ghz 24-Core 256GB P440ar
$302.90
Lenovo ThinkServer TS140 Xeon E3-1225v3 3.20GHz 16GB RAM Tower Server Needs HDD
$99.99
Cisco UCS C240 M4 2U Server w/ 2x E5-2680v4, NO RAM
$39.99
Dell Poweredge R730xd LFF 14-Bay 2U Server | Choose Your CPU & RAM Config
$454.99
1U BareMetal pfsense opnsense Router Firewall DNS Server 6x 10GB Ethernet Ports
$149.00