First of all, this would have been better to place in the Computer Security section of the forum.
However...
1 - If you are behind a router you are most likely behind a NAT which is basically a hardware firewall.
2 - If you installed one, obviously you are. They could be seen in the taskbar.
3 - http://www.canyouseeme.org/ Go there, type different ports...if it ever does NOT say "Error: I could not see your service on xx.xxx.xxx.xxx on port (xx)" then the site was able to access an open port. (Which probably means you don't have a good firewall)
Hint: on that web page, make sure you try port 80.