I need to apply port address translation to my LINUX Firewall to all internal traffic destined for the outside and beyond.

Whilst I have managed to find an IPtables script to set this up, I need to find the best way of testing if the firewall is correctly configured, so that all internal traffic passing through the firewall has the ip address of the firewalls external interface.

Likewise I need to test that two simultanous connections can be made to the outside world from the internal network - and that they share the same IP address but have different port numbers.

Can someone give me some advise on the best thing to use to test the above? I need to document my findings in a report. Would I use nmap scans? I dont really know, and searching on Google hasn't really helped.

Thanks in advance!