Efficiently test if a port is open on Linux?

Question

From a bash script how can I quickly find out whether a port 445 is open/listening on a server.

I have tried a couple of options, but I want something q

Answer 1

nc -l 8000

Where 8000 is the port number. If the port is free, it will start a server that you can close easily. If it isn't it will throw an error:

nc: Address already in use