Gary, all versions of Windows create environment variables that determine what kind of processor architecture is running. If it is a 64 bit process you will always find PROCESSOR_ARCHITEW6432 in the environment variables, however, it will be nonexistant on 32 bit operating systems, therefore you can readily tell if the OS is running a 64 bit process or 32 bit. All windows will have the environment variable PROCESSOR_ARCHITECTURE, but you would need to verify the value against a set of unknowns.
The mere existance of PROCESSOR_ARCHITEW6432 assures you that you have a 64 bit OS running so the result would be effective.