ports
 

how can I find out what ports are being blocked by a firewall?
e.g. I want to check at school what ports are open

put up an FTP @ home, open on all ports , and try connecting oneport at a time.

Fastest way I know.

There are portscanners out there... don't know how they work however.

Portscan will be the fastest way, and there are a lot of freeware portscanners to be found :)