RHEL / CentOS / Debian Linux Detecting Rootkits

A rootkit is a program designed to take fundamental control of a
computer system, without authorization by the system’s owners and
legitimate managers.

Most rootkits use the power of the kernel to hide themselves, they
are only visible from within the kernel. How do I detect rootkits under
CentOS or Debian Linux server?

Linux Detecting / Checking Rootkits with Chkrootkit and rkhunter Software