You may use netstat command, which print network connections, routing tables, interface statistics, masquerade connections, and multicast memberships etc. Another (and suggested) option is to use lsof command, which list open files, and ports on Linux, FreeBSD, Solaris and other Unixish systems. You must run this by login as root user on your server:

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

To see FQDN (Full DNS Hostname), try removing the -n flag:

netstat -vat

Examples – lsof Command

To display the list of open ports, enter:

lsof -i

To display all open files, use:

lsof

To display all open IPv4 network files in use by the process whose PID is 9255, use:

lsof -i 4 -a -p 9255

For FreeBSD Users:
Use the sockstat command on FreeBSD as follows:

sockstat
sockstat -l
sockstat -4 -l
sockstat -6 -l