Well, either way, the NetBSD CPU probe report notes it under 
"extensions" at boot time, e.g.

DIGITAL Server 3000 Model 3305 6500A, 500MHz, s/n NI82407HTJ
8192 byte page size, 1 processor.
total memory = 512 MB
(2096 KB reserved for PROM, 509 MB used by NetBSD)
avail memory = 364 MB
using 16300 buffers containing 127 MB of memory
mainbus0 (root)
cpu0 at mainbus0: ID 0 (primary), 21164A-2
cpu0: Architecture extensions: 1<BWX>
[...]

	Erik <fair@clock.org>