The best way is to look in your httpd.conf file and see what the default is. It could also be overridden by your specific virtual host. I start by looking at /etc/httpd/conf/httpd.conf
or /etc/apache2/httpd.conf
and search for error_log. It could be listed as either /var/log/httpd/error_log or /var/log/apache2/error_log
but it might also be listed as simply logs/error_log
.
In this case it is a relative path, which means it will be under /etc/httpd/logs/error_log
. If you still can't find it check the bottom of your httpd.conf file and see where your virtual hosts are included. It might be in /etc/httpd/conf.d/<- as "other" or "extra". Your virtual host could override it then with ErrorLog "/path/to/error_log".