Wade just asked me about this and it took me a while to find so I'm posting it here for _post_erity: The CPU flag that tells you if your processor is 64-bit (aka EM64T) capable is
lm. It stands for Long Mode, where 16-bit is Real Mode and 32-bit is Protected Mode. This is important because many manufacturers are shipping boxes as 32-bit even though they can do 64. Generally it's because they have to ship a 32-bit version of Windows™ for better hardware support and they don't want you to be upset about it.
In Linux you can see this by running
cat /proc/cpuinfo and looking for