If you are running on a distro that uses RPM (Red Hat, Mandrake, SuSE,
etc), then you can use RPM to tell you. At a shell prompt, type:
`rpm -qa | grep kernel`
and look at the output. You should see a package that corresponds
to your kernel (often named something like kernel-2.4.18-3)
and a kernel source package with the same version (often named
something like kernel-source-2.4.18-3). If none of the lines seem
to correspond to a source package, then you will probably need to
install it. If the versions listed mismatch (ex: kernel-2.4.18-10 vs.
kernel-source-2.4.18-3), then you will need to update the kernel-source
package to match the installed kernel. If you have multiple kernels
installed, you need to install the kernel-source package that
corresponds to your *running* kernel (or make sure your installed
source package matches the running kernel). You can do this by
looking at the output of 'uname -r' and matching versions.