How do I tell if my application is running as a 32-bit or 64-bit application?

if (IntPtr.Size == 8) 
{
    // 64 bit machine
} 
else if (IntPtr.Size == 4) 
{
    // 32 bit machine
}

I found this code from Martijn Boven that does the trick:

public static bool Is64BitMode() {
    return System.Runtime.InteropServices.Marshal.SizeOf(typeof(IntPtr)) == 8;
}