How can I tell the difference between a J.I.S. and ISO square taper bottom bracket? I have a square taper bottom bracket. How can I tell whether it is a J.I.S. or ISO taper with a minimum amount of disassembly?

FWIW, it's a Ritchey from 2004. However, it would be nice to get answers that apply to other brands as well.
The width across the narrowest part of the flats (end of spindle):

JIS = 12.65mm

ISO = 12.33mm

Use calipers.