Securing your systems is a mandatory task for any mission critical system. However, regardless how hard you try, it is impossible to guarantee that the system is not compromised. When administering important servers, where the integrity and security of your data is critical, it is a good idea to do some extra checks from time to time to ensure that the system is still under control of the administrator.
An easy check that often can reveal unwanted changes can be done by means of rpm. The package manager has a built in verify function, that checks all the managed files in the system for changes. To do a verify of all files, run the command rpm -Va. However, this command will also display changes in configuration files and you will have to do some filtering to detect important changes.
An additional problem to the method with rpm is that an intelligent attacker will modify rpm itself to hide any changes that might have been done by some kind of root kit which allows the attacker to gain control over your system. To solve this, you should implement a secondary check that can also be run completely independent of the installed system. This is where AIDE comes into play.