It seems no one has asked this question before (most regard host-based firewalls).

Anyone familiar with port scanning tools (e.g. nmap) knows all about SYN scanning, FIN scanning, and the like to determine open ports on a host machine. Question is though, how do you determine the open ports on a firewall itself (disregard whether the host you're trying to connect to behind the firewall has those particular ports open or closed). This is assuming the firewall is blocking your IP connection.

Example: We all communicate with serverfault.com through port 80 (web traffic). A scan on a host would reveal port 80 is open. If serverfault.com is behind a firewall and still allows this traffic through, then we can assume the firewall has port 80 open also. Now let's assume the firewall is blocking you (e.g. your IP address is under the deny list or is missing in the allowed list). You know port 80 has to be open (it works for appropriate IP addresses), but when you (the disallowed IP) attempt any scanning, all port scan attempts on the firewall drop the packet (including port 80, which we know to be open). So, how might we accomplish a direct firewall scan to reveal open/closed ports on the firewall itself, while still using the disallowed IP?