How to list the ports open and closed by my firewall?

[frankpeng-VsqqI1RANlHk1uMJSBkQmQ at public.gmane.org]

Hi, 
 
I made a simple firewall and I did not block some ports but some clients says my firewall blocked some ports. How do I know which ports are open and which are closed by my firewall?
 
Thanks.
 
Frank Peng. 
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