Basically all ports will be blocked on your router unless you enabled port forwarding & mapped a internal IP & a service.If a port is not mapped then the router should drop traffic for the specified socket pair [a combination of a IP address & port].
Ports are said to be open if & only if you have some kind of service running.By service I mean a HTTP server or a FTP server.
There are two ways as I can see it either use a similar service provided by steve gibson GRC Shields UP though I've yet to figure out how it works exactly & how far it is accurate, or use a port scanner such as nmap & scan your IP from outside the private lan.
This should give you a fair idea about open/closed ports.Hope this helps.
