Not sure it's 100%, but it worked in my VMWare test sessions.

Code:
Break On
 

if isVMWare
  ? "VMWare Session"
else
  ? "Not VMWare Session"
endif
 

 

function IsVMWare
  dim $objWMIService, $colItem, $objItem
  $objWMIService = GetObject("winmgmts:\\.\root\cimv2")
  $colItems = $objWMIService.ExecQuery("Select * from Win32_BIOS")
  For each $objItem in $colItems
    if instr($objItem.SerialNumber,"VMWARE")
      $ISVMWare=1
    endif
  Next
Endfunction