Here comes a very very very short tip to detect whether the server is behind the firewall or not. Remember nmap that you have used a bit in RHCE Training. Using the same nmap you can do this.
What you do next is up to you. 🙂
Using – nmap -sA x.x.x.x
Here comes the illustration from my machine. If the result comes as “filtered” that means its protected and is behind the firewall and if it comes “unfiltered” … well you know … its not behind any firewall.