The tool chkconfig does this for you. You can list all of the services and what they do at each runlevel by typing:
bash# chkconfig --list
This will list a lot of services so you might have to scroll through the list. If the xinetd service is on then you will get a list of all of the subservices that it provides. All of these services will run at the same runlevel as xinetd.
If I wanted a the mail server to run on startup I would type this:
bash# chkconfig sendmail on
This will start sendmail at the runlevels and the order listed at the top of the /etc/rc.d/init.d/sendmail file. The line looks something like this:
# chkconfig: 2345 80 30
This means it will run at levels 2, 3, 4, and 5. It will start as number 80 and stop as number 30.
For more information about chkconfig try either typing the name or looking at the manual page:
bash# chkconfig bash# man chkconfig