Solved Long mode

A bit in the CPUID extended attributes field informs programs in real or protected modes if the processor can go to long mode, which allows a program to detect an x86-64 processor.

Or, just look up the exact type of CPU and see if it supports Intel64 (AMD64, EM64T, x86_64, IA-32e).
I'm disappointed to find that my ThinkPad T60 doesn't support long mode, which I found by PXE booting mfsBSD...

The BIOS tells me that is has a Genuine Intel (R) CPU T2300. Not sure what other information I can glean from the BIOS.

I notice that IVT is disabled, not that I know what that does. Maybe I should enable it to see what it does do.
How can I tell if a particular computer supports long mode?

Boot off any LiveCD (Linux or FreeBSD) and check the dmesg(1) output for the Features line that describes all the x86 extensions that are supported. If you see LM (Long Mode) in the list, then it supports it. If you don't see LM, then it doesn't support it. On a Linux system, you can also cat /proc/cpuinfo to get that same info.

Or, just try to boot off a 64-bit LiveCD or install CD, and it'll tell you quite quickly whether or not Long Mode is supported. :)