1) If you've cPanel installed on the server then login to the WHM > Server Status > Service Status option which will provide you entire system information (Disk Space, RAM, CPU, Server Load and current status of all service.
2) You can also check the memory usage using command line option by running free command.
3) vmstat is another command displays current virtual memory usage- processes, memory, paging, disks, and CPU information.
4) To check currently running processes on the server fire ps aufx command.
5) The top (or htop) command supports real-time monitoring of server resource usage.
I hope it helps. 🙂