SUSE Open 10.3
Is there a way to see exactly what external ports are open in a software firewall?

Try using nmap.

I don't know how to use it, can you explain?

scroll down to "examples"
That way then..

If you refer to iptables, a software firewall installed on the machine you're working on, then the answer is: everything that's not specifically declared to be something else than policy, is dealt with as the policy says. Or "if policy is to drop/deny, everything is dropped that is not explicitly allowed", or "if policy is to allow, then everything is allowed that is not explicitly denied".
shows you the current iptables configuration. Empty tables mean no rules, which means no policies, which means everything can go without restrictions.

Just as a note, I don't always get what people refer to with "software" firewalls (or their counterparts, "hardware" firewalls). After all if the bits are inspected and dealt with, it's software that does it - a hardware firewall would mean, to me, a physical machine that would inspect the currents that run trough the cables, or light if optics is used instead, and then either stop it from getting where it wants or allowing it to continue. As most firewalls probably don't ask the electrons or photons where they might be going, I consider most firewalls software firewalls even if they carry a "Cisco" logo and come in heavy green steel boxes.

Hate to be pestering you but when I tried to run nmap -sS -O scanme.nmap.org/24 I get a command not found.

try downloading it then?

http://insecure.org/nmap/

and when you got it, just do a:
best do that as root, since root does a SYN scan (that's the -sS flag, no need to supply it if you're root) by default, instead of the ugly connect() scan used by normal users