How to determine the application listening to a particular port

For example, I run nmap and find that port 712 is open:

712/tcp    open        unknown

But it's easily identified:

# fuser 712/tcp
712/tcp:  2098

# ps -fp 2098
     UID    PID    PPID  C    STIME TTY    TIME   CMD
    root   22098   8999  0    Aug18 ?      22:33  /usr/etc/customd

# man -k customd
custom (1M)           - Custom daemon