How can I check to see which TCP ports are in use?

A. The Netstat command can list currently used ports, which might be helpful if you suspect an application is clashing with another one on an active port. Use the -an switch to show all connections and listening ports in numeric form.

C:\> netstat -an

Check the output for any port that you think your program might use.

Take a look at using the Port Reporter. Also read: TCP vs. UDP Ports.