You can use the following options to find the document root of a domain on your cPanel server.
With root SSH access
If you have root access to the server you can use grep the domain out of the userdatadomains file to quickly find the document root.
grep example.com /etc/userdatadomains
IMPORTANT: Do not make any edits to the userdatadomains file.
With cPanel GUI access
If you are logged into the cPanel account, you can find the document root for a domain within the Domains icon which is under the Domains section.
The home icon before the document root represents the home directory portion of the path. The document root shown there is a link that you can click which will open the file manager inside of the document root. When looking at the left sidebar, you can see the exact path that the home icon represents.