How do I find out if I am behind a firewall?

if the router is reporting your WAN ip as your public IP then you have a public IP assigned to you.

If you want to know who the provider is you can do a lookup as to who the owner is ARIN's lookup site if you are in the US.

As far as if you are behind a firewall - that will be more difficult since even if you did something like an nmap scan your belkin would probably drop the packets anyway.

You should also note that running servers is against every TOS i've seen on consumer level internet connections, and some providers (Verizon i'm looking at you) will filter common server ports (25, 80, 443, 110, etc)

Honestly if you want to run a web server the least headache would be to shell out the 10$ or so a month for a hosted website.