Would love to know if there's a way to use some control utility to find it out. Doesn't seem like apachectl can do it, but maybe it can.
Edit: I should probably add that it's
root# cat /etc/redhat-release
CentOS Linux release 7.4.1708 (Core)
root# httpd -v
Server version: Apache/2.4.6 (CentOS)
Server built: Oct 19 2017 20:39:16
and that the "default" DocumentRoot as specified by apache docs '' does not exist
# ls -la /usr/local/apache/htdocs
ls: cannot access /usr/local/apache/htdocs: No such file or directory