"showmount: command not found": how do I know what to install?

You can use apt-file for that (you might need to install it):

apt-file search showmount

This reveals that the command is in the nfs-common package.

Typically when you're looking for a binary you can restrict the search by prefixing the binary with bin/:

apt-file search bin/showmount

To install apt-file, run

sudo apt-get install apt-file
sudo apt-file update

If you end up with apt-file 3.0 or later, you won’t need to update the indexes again separately (after the initial download above), they are updated whenever the main APT indexes are updated.

You might want to ease your life by installing the command-not-found helper

$ sudo apt install command-not-found
[…]
$ showmount
Command 'showmount' is available in '/sbin/showmount'
The command could not be located because '/sbin' is not included in the PATH environment variable.
This is most likely caused by the lack of administrative priviledges associated with your user account.
showmount: command not found

You can use apt-cache, which is included as default on debian based distributions (afaik):

apt-cache search showmount

which gives the result nfs-common as well.