From Fedora Project Wiki
http://people.redhat.com/~rjones/virt-what/
virt-what lets you detect if you are running inside a VM.
Simply install virt-what in a guest (or on the host if you like), and run it as root:
# virt-what kvm
Consult the virt-what man page to find out the possible values that it can print:
$ man virt-what