The size of the System.IntPtr will tell the bitness of the application, but not necessarily the platform. A 32-bit application can run on a 64-bit platform and IntPtr will be 4 bytes. I believe that window.navigator.platform and cpuClass will both have
the same issue: they will tell if the host application is 32-bit or 64-bit, but not if the OS is 32-bit or 64-bit.
Depending on your specific circumstances that may be sufficient, so long as you can avoid running in 32-bit on 64-bit OSes.
--Rob