Determine if current PowerShell Process is 32-bit or 64-bit?

If you're shell is running on .NET 4.0 (PowerShell 3.0):

PS> [Environment]::Is64BitProcess
True

You can use this as well. I tested it on PowerShell version 2.0 and 4.0.

$Arch = (Get-Process -Id $PID).StartInfo.EnvironmentVariables["PROCESSOR_ARCHITECTURE"];
if ($Arch -eq 'x86') {
    Write-Host -Object 'Running 32-bit PowerShell';
}
elseif ($Arch -eq 'amd64') {
    Write-Host -Object 'Running 64-bit PowerShell';
}

The value of $Arch will either be x86 or amd64.

EDIT:

The caveat is that Process.StartInfo.EnvironmentVariables always returns the environment of the current process, no matter which process you execute it on.

To determine in your script what version of PowerShell you're using, you can use the following helper functions (courtesy of JaredPar's answer to an related question):

# Is this a Wow64 powershell host
function Test-Wow64() {
    return (Test-Win32) -and (test-path env:\PROCESSOR_ARCHITEW6432)
}

# Is this a 64 bit process
function Test-Win64() {
    return [IntPtr]::size -eq 8
}

# Is this a 32 bit process
function Test-Win32() {
    return [IntPtr]::size -eq 4
}

The above functions make use of the fact that the size of System.IntPtr is platform specific. It is 4 bytes on a 32-bit machine and 8 bytes on a 64-bit machine.

Note, it is worth noting that the locations of the 32-bit and 64-bit versions of Powershell are somewhat misleading. The 32-bit PowerShell is found at C:\Windows\SysWOW64\WindowsPowerShell\v1.0\powershell.exe, and the 64-bit PowerShell is at C:\Windows\System32\WindowsPowerShell\v1.0\powershell.exe, courtesy of this article.