What ports are being listened on?

Here are some examples of how to find out on a linux based system.

sudo lsof -i -P -n | grep LISTEN
sudo netstat -tulpn | grep LISTEN
sudo lsof -i:22 ## see a specific port such as 22 ##
sudo nmap -sTU -O IP-address-Here

Reference: https://www.cyberciti.biz/faq/unix-linux-check-if-port-is-in-use-command/