Shared and Reseller
You can login to cPanel and click the Error Log icon. This contains the last 300 Apache errors triggered by your website.
VPS and Dedicated Server
On a Dedicated Server or VPS without a cPanel installation, most log files would be located in the /var/log/ directory. The Apache log would then be located in the /var/log/httpd/ directory.
However, for all of our servers with cPanel installs, the actual location of the Apache error log is /usr/local/apache/logs/error_log, where "error_log" is simply a plain text file.