Aside from what your Alpha reports at boot time, e,g.

	NetBSD 1.6.2_STABLE (DS3305) #9: Wed Mar 17 01:23:20 PST 2004
	    root@digital.clock.org:/usr/obj/sys/arch/alpha/compile/DS3305
	DIGITAL Server 3000 Model 3305 6500A, 500MHz, s/n NI82407HTJ
	8192 byte page size, 1 processor.
[...]
	cpu0 at mainbus0: ID 0 (primary), 21164A-2
	cpu0: Architecture extensions: 1<BWX>

there is also the intro(4) man page that I tried to make as complete
as possible. The BWX extensions are in the 21164-A (EV56), and the
21264 (EV6 and later), but not in the 21164 (EV5), so far I as I
was able to determine...

BTW, if anyone has data on the 21264 CPUs (and later) and the
systems that use them, I'd love to fill out the later information
in Alpha's intro(4).

	Erik <fair@netbsd.org>