Various ways of detecting rootkits in GNU/Linux

Consider this scenario… Your machine running GNU/Linux has been penetrated by a hacker without your knowledge and he has swapped the passwd program which you use to change the user password with one of his own. His passwd program has the same name as the real passwd program and works flawlessly in all respects except for the fact that it will also gather data residing on your machine such as the user details each time it is run and transmit it to a remote location or it will open a back door for outsiders by providing easy root access and all the time, you will not be aware of its true intention. This is an example of your machine getting rooted – another way of saying your machine is compromised. And the passwd program which the hacker introduced into your machine is a trojaned rootkit.