Common locations for your apache log-dir are:

• /home/httpd/logs/
• /var/log/httpd/
• /usr/local/apache/logs/

And it wouldn't hurt actually searching for it yourself: find / -name error_log -print

update
Read jcwren's post below and ye shall learn how to harness the power of Grep, for it is truly powerful!

[ar0n]