How to: Tell if a PowerShell script is running as the Administrator

$currentPrincipal = New-Object Security.Principal.WindowsPrincipal([Security.Principal.WindowsIdentity]::GetCurrent())
if (-not ($currentPrincipal.IsInRole([Security.Principal.WindowsBuiltInRole]::Administrator)))
    Write-Warning "This script needs to be running as the administrator."
    Exit 1

Write-Host "You are running as the administrator."

This script gets the current Windows Identity, then queries it to find out if it has the appropriate role.