open ports knowledge

 https://askubuntu.com/questions/9368/how-can-i-see-what-ports-are-open-on-my-machine

on remote server you can list ports open using 

sudo netstat -ntlp | grep LISTEN

However, even if you opened ports, they might not be open to other servsers aka outside world due to additional firewall restriction or centralized control,

you can check default firewall policy by using sudo ufw status, but it might be subject to outside control

on other server simply type

nmap <your_server>

sudo apt install nmap