If you are using CentOS the answer is pretty simple:

[codesyntax lang="bash"]

chkconfig --list | grep $(runlevel | awk '{ print $2}'):on

[/codesyntax]

But how about Debian Linux? How is this done?

[codesyntax lang="bash"]

R=$(runlevel | awk '{ print $2}')
for s in /etc/rc${R}.d/*; do basename $s | grep '^S' | sed 's/S[0-9].//g' ;done

[/codesyntax]

If you like - and I am sure most of you will - you can use rcconf, Debian Runlevel configuration tool.