We do this with a quick and dirty VBS script. We just run it within a batch file and check the error level that's returned.

We've tested it under ESX 1.5.2 and 2.0.1, but I assume it would work just fine in the workstation version as well.

' *******************************************************************
' Filename: isVMware.vbs
'
' Dependencies: none
'
' Queries WMI for the Manufacturer value. If it contains "VMware," the
' script returns errorlevel=1. Otherwise the script returns
' errorlevel=0
'
' *******************************************************************

set objNet = WScript.CreateObject("WScript.Network")

strWinMgt = "winmgmts://" & objNet.ComputerName

set DeviceSet = GetObject(strWinMgt).ExecQuery("select * from Win32_ComputerSystem")

i=0

for each Device in DeviceSet
if Instr(1, Device.Manufacturer, "VMware", 1) then
i=i+1
end if
next

WScript.Quit(i)