Which .NET version is my PowerShell script using?

On PowerShell 2.0, just take a peek at the $PSVersionTable variable:

PS> $psversiontable

Name                           Value
----                           -----
CLRVersion                     2.0.50727.4927
BuildVersion                   6.1.7600.16385
PSVersion                      2.0
WSManStackVersion              2.0
PSCompatibleVersions           {1.0, 2.0}
SerializationVersion           1.1.0.1
PSRemotingProtocolVersion      2.1

On PowerShell 1.0, use [System.Environment]::Version:

PS> [Environment]::Version

Major  Minor  Build  Revision
-----  -----  -----  --------
2      0      50727  4927