I had to create a script that was only supposed to run on a virtual machine.  It needed to detect at least Hyper-V and VMWare, but I was about to get VirtualBox for free...

Here is the code

 (gwmi win32_computersystem | select -expand model) -ilike "*virtual*"

Note the use if the "-ilike" operator.  The -match operator can have some unintended consequences if you don't keep in mind that you are dealing with a regular express.  -ilike is case independent and uses the very familiar "*" character as a wildcard.

You can modify the code to set a boolean variable like this:

$IsVirtual = [bool]((gwmi win32_computersystem | select -expand model) -ilike "*virtual*")

Or create a function like this:

Test-IfVirtualMachine {
[bool]((gwmi win32_computersystem | select -expand model) -ilike "*virtual*")
}

Drop a line if you know of other hypervisors that contain "virtual" in their computer model!