$ man cd
No manual entry for cd
But what if no man page is available for that command?
This could be due to a number of different reasons. For example, the Linux system was originally installed without any man pages at all. I worked with a Linux-based mobile gateway device that has 0 man pages pre-installed. This is to save disk space on a 1 GB Solid State Drive (SSD). In most typical desktops or servers, man pages are pre-installed.
A man page can be missing for a particular command because the administrator, for whatever reason, did not install the man page for that command. If that is the case, google is your best friend. The challenge here is to narrow down the search to get to the man page quickly.
There is yet another reason why there is no man page for a command. If it is a bash shell built-in command, it will NOT have its own man page. Documentation is still available but it is a bit harder to get to it. It turns out that information about bash built-in commands can be found inside the bash man page. You simply enter man bash, and search for the SHELL BUILTIN COMMANDS section (using the command /^SHELL BUILTIN). Then scroll down until you reach the built-in command you are looking for.
$ man bash
How do you know if something is a bash built-in command in the first place?
Use the type command (another bash built-in).
$ type cd
cd is a shell builtin
Note that not all Linux distributions behave the same way when you man a bash built-in command. Debian Etch, the distro I use at home, reports no man page when you man cd. Some distributions, like the Red Hat-based Centos, will display the bash man page. In this case, you are one step ahead of the rest.