Some useful netstat commands:
-l or --listening shows only the sockets currently listening for incoming connection.
-a or --all shows all sockets currently in use.
-t or --tcp shows the tcp sockets.
-u or --udp shows the udp sockets.
-n or --numeric shows the hosts and ports as numbers, instead of resolving in dns and looking in /etc/services.
netstat -atn # check tcp ports open
netstat -aun # check udp ports open
netstat -atun # check both udp and tcp ports open