First SSH into the server.
Once logged in you can type "apt list" to see all the python modules installed.
Lets say we wanted to find out what version of "wkhtmltopdf" is installed on the server (this is the module that generates pdf reports). We can type "apt search wkhtmltopdf" and it will show us all of the python modules that have that string in its name.
This is useful for trying to see what version of python modules we have installed on the server and troubleshooting what could be causing issues. For instance the recommended version of wkhtmltopdf for Odoo11 is 0.12.1 if there was some issues with the pdf reports we could uninstall the higher version and install the recommended version on the server.