What is listening at given port?
Solution 1:
netstat -lp
is the command you want, but you need to be root to see all the data.
Solution 2:
ss -tlp
Quicker than netstat.
Solution 3:
Take a look at this and this page.
Solution 4:
Already answered, but if it's on a system you don't have a shell on, running nmap -sV -p <port number> <target> will often give you the answer. Add -sU if scanning a UDP port.