There are basically 2 ways of seeing if the firewall is configured. You should use both of the methods and verify that the firewall is both configured and configured the way you wish it to be.
First, check that the firewall rules have been applied. Pretty much all modern Linux firewall solutions use iptables for firewall. You can see that there are rules in place with iptables command:
This will return the current set of rules. There can be a few rules in the set even if your firewall rules haven't been applied. Just look for lines that match your given rulesets. This will give you an idea of what rules have been entered to the system. this doesn't guarantee the validity of the rules, only that they have been understood.
Next, you will use a second computer to test for connections against the host in question. This can be easily done with the
nmap command (found in nmap package). Quick and dirty way of checking is:
nmap -P0 10.0.0.10
nmap -P0 -sU 10.0.0.10
Replace the IP address 10.0.0.10 with your destination hosts IP address.
The first line will scan for TCP ports that are open and available from the second computer. Second line will repeat the scan but this time with UDP ports. -P0 flag will prevent the host from being tested with a ICMP Echo packet, but might be blocked by your firewall rules.
The scan might take a while so be patient. There is also a GUI frontend for nmap called
zenmap which makes it a bit easier to interpret the scan results if there is a lot of output.