finding unused vlans assigned to a port

[troy_alan]

Is the "show fdb port x" command a fair way to figure out what vlans don't need to be tagged on that port?

[jeronimo]

If the FDB does not contain an entry for a VLAN then that all devices on that VLAN on that port are currently quiet (or there are no devices). So in short no, it is not a 100% guarantee.