Reasons to Want to Detect a Firewall
The need to detect a firewall is not only based on curiosity or on the desire to do harm (i.e. to hack a computer). There are many cases when you have a legitimate reason want to gain remote access through a firewall and to try to detect a firewall.
For instance, a firewall might interfere with the normal use of a legitimate application. In this case, you need to detect the firewall because very often if there is a firewall enabled, it blocks certain ports and the application cannot function properly. This often happens silently and the firewall doesn't tell you about its existence.
Another legitimate reason to detect a firewall is for performing a security check. If you are performing a security check, your mission will be to see if the firewall is up and running. You might also try to make sure that the firewall cannot be easily detected because if it can, this makes it easier for hackers to try to disable it.
Steps to Detect a Firewall
Sometimes firewalls show their presence and you do not have to perform miracles in order to detect a firewall. If you are examining your own computer for a firewall, you have more options, especially if you have administrative privileges. In this case, you should first look in the system tray and check if any of the applications there could be a firewall. Place the mouse over an icon to see the name of the program it represents. If the name of the program means nothing to you, search with Google to see what this program is supposed to do.
The second approach to detect a firewall is to check what programs are running on your computer and learn whether one of them is a firewall. On Windows, you can do this if you press Ctrl+Alt+Del to open the Windows Task Manager. Go to the Processes tab and see if the list of running processes includes something that could be a firewall. Unlike malware, firewalls do not object being listed as a process, so it is quite possible that if there are one or more firewalls running on your computer, you will see it listed.
Finally, if you cannot detect a firewall in any other way, you can try a port scan. You can perform a port scan on your own computer or on a remote system. However, be warned that if you do a port scan on a remote system without the explicit authorization of the administrator, this could get you into trouble. A port scan might not tell you directly which ports are blocked. However, if there are blocked ports, this is a hint that there must be a firewall on the remote system.
It might be hard to detect a firewall, but if you notice that some applications are denied Internet access, this is a good indicator that a firewall is preventing them from accessing the Internet. In such cases, you may need to hunt until you manage to find the firewall(s) and unblock the applications. I recall a case when a customer complained that a product was not working. As it turned out, he had three or four firewalls and our support department had a hard time detecting and configuring them all to allow the application. Therefore, detecting a firewall could be a bit of a challenge even for an experienced pro!