In the Webmin GUI, go to Servers -> Apache Webserver.
On the 'Existing Virtual Hosts' tab, look at the 'Document Root' for 'Default'.
Another way is to check your DocumentRoot setting in httpd.conf. Not sure what OS you have installed, but for Webmin on CentOS it is located at /etc/httpd/conf/httpd.conf. Look in the file for something like:
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