Here's a quick powershell snippet to display the Windows groups setup as logins on SQL Server.

Import-Module SQLPS -DisableNameChecking -ErrorAction Ignore;
Import-Module ActiveDirectory -DisableNameChecking -ErrorAction Ignore;
$sql_server = "sql_instance";
$srv = New-Object Microsoft.SqlServer.Management.Smo.Server $sql_server;
$srv.Logins | Where-Object {$_.LoginType -eq "WindowsGroup";};

Output will be similar to below.

Name                                          Login Type    Created
----                                          ----------    -------
NT SERVICE\ClusSvc WindowsGroup 13/10/2013 11:17 NT SERVICE\MSSQLSERVER WindowsGroup 13/10/2013 12:17 NT SERVICE\SQLSERVERAGENT WindowsGroup 13/10/2013 12:17 Domain\Group 1 WindowsGroup 04/06/2013 12:29 Domain\Group 2 WindowsGroup 02/04/2013 11:25 Domain\Group 3 WindowsGroup 02/04/2013 12:22