Just because the OS is 64 bit, doesn't guarantee how the hardware will
handle memory. For example, you can run a 64 bit OS just fine, with only
1GB of main memory present. When the CPU is capable of 64 bit instruction
execution, it doesn't magically change the amount of memory that the
Northbridge supports. The Northbridge limits remain at the values
stated in the original manufacturer information.
If the processor was known to be an Intel product, then Intel provides
a processor identification utility. It is possible the X86-64
attribute may be recognized by a utility like that.