Keeping tabs on your server resources means checking on memory usage from time to time. There are a few different ways of accomplishing this, depending on your preference and system access.
Login to WHM and go to WHM, Server Information (or Service Status).
There you'll see a section on Memory Information containing memory usage details.
SSH into the server and login.
Commands relating to memory usage include….
You'll get much more than just memory information with top, which is a command giving real-time details on resource usage.
Press m or shift-m to get memory related information such as per-process memory consumption.
Press q to quit and return to the prompt.
A much more streamlined output of details, free will give you insights into memory usage such as physical, buffers+cache and swap (amongst other things).
If you'd like to simplify the display further, when you run the free command, just add a parameter like "free -m" for display in megabytes or "free -g" for display in gigabytes.