For both Windows and Linux, the command is 'netstat'. To find out if you have any open ports (all PCs do to some extent), you can do a 'netstat -na' and look for any lines that have the word "LISTEN"...
The fancy way is to use an expensive peice of equipment that sends tones through a network jack that can be scanned by another peice of equpment near the switch. Another way is to have a helper plug...