What is Netstat? — is a common command line TCP/IP networking utility available in most versions of Windows, Linux, UNIX and other operating systems.
Netstat provides information and statistics about protocols in use and current TCP/IP network connections.
netstat command to find open ports.
# netstat --listen
To display open ports and established TCP connections, enter:
$ netstat -vatn
To display only open UDP ports try the following command:
$ netstat -vaun
If you want to see FQDN (full dns hostname), try removing the -n flag:
$ netstat -vat
There are number of tools available today, however, these commands are what I usually use in daily basis.
I hope this will help you out in some ways.