There are two common ways to list all processes on a system. Both list all processes running by all users, though they differ in the format they output (the reason for the differences are historical).
ps -ef # lists all processes ps aux # lists all processes in alternative format (BSD)
This can be used to check if a given application is running. For example, to check if the SSH server (sshd) is running:
ps -ef | grep sshd