Find out if the OS is running in a virtual environment

Linux adds the hypervisor flag to /proc/cpuinfo if the kernel detects running on some sort of a hypervisor.

On Linux you can use the command virt-what

[root@myhost]# virt-what
vmware

Example Output for a linux on Windows HyperV

[root@linuxvm11~] # virt-what
hyperv

This worked better for me as it gives me specific information about the manufacturer and the product name.

dmidecode -t system|grep 'Manufacturer\|Product'

Output on Dell server:

Manufacturer: Dell Inc.
Product Name: PowerEdge C5220

Output on Virtualbox VM:

Manufacturer: innotek GmbH
Product Name: VirtualBox

Output on KVM/QEMU:

Manufacturer: QEMU
Product Name: Standard PC (i440FX + PIIX, 1996)

This is great for scripts that can parse these out for better identification of servers... but if you use Chef in your infrastructure, you can check the node attribute Virtualization -> system in the chef server .