As far as I know by default Apache will listen to all interfaces, but yes do check it as The Cog mentions. Also, we can tell from the access.log file that it is listening to the NIC.
The file httpd.conf is now empty, and only there for history reasons, I think. The listening stuff is taken care of in /etc/apache2/sites-available/default and /etc/apache2/ports.conf
To observe if the packets are even arriving at the server computer you could run a packet sniffer:
sudo tcpdump -n -tttt port 80