Digital BW, The Print

I use the Sunny16 or Basic Daylight Exposure (BDE) rules to test true
ISO. That means
exposing at ƒ16 by using the true film speed as the shutter speed
to check true speed with respect to 
processing. I go downtown between 10:00 and 2:00 on a sunny day, go
to my usuall test block  (a mix of buildings with average tones, not
a bunch of all black glass, nor all white 
buildings) with the sun behind me, and shoot 3 sheets  of chrome at 
ƒ/16 @ 1/60, ƒ/16-1/3 @ 1/60 and ƒ/16-2/3 @1/60. For Fuji
Provia, my local E-6 lab renders the sheet 
exposed at ƒ16 @ 1/60 as correctly exposed, meaning the effective
speed at that lab is ISO 64. When I use the Milwaukee lab, the sheet
exposed at ƒ16-1/3 @ 1/60 is correctly 
exposed, meaning the effective ISO at that lab is ISO 80. (I've never
seen an ISO 100 film actually be ISO 100) 

You could do the same for digital. A lot of it will be determined by
your camera software's selected colorspace, sRGB, Adobe RGB,
ColorMatch RGB, etc,
Photoshops edit space
you have selected in your color settings, if you are properly using
Photoshops color management with respect to assigning and/or
converting to profile, 
your monitors calibration, and the actual cameras software. If the
camera is supposed to be an ISO 100 chip, expose a frame under the
above conditions at  ƒ16 @1/100 
( or ƒ16-2/3 @ 1/60) and see how the image looks. Use the
densitometer in photoshop to check highlights and shadows.

I often see a difference between in-camera meters (Reflected light)to
hand held meters (Incident light. Spot meters are another matter).
The camera meter is reading the actual 
reflected light values and if you have a tricked out matrix metering
sensor, it will compensate for unusually bright areas or unusually
dark areas and give you a pretty good exposure 
based on a number of predetermined situations programmed into the
sensor. 
The hand held meter
only reads light falling on the dome and does not provide any
relevant data
regarding how much light is actually being 
"reflected off the subject" into your camera. 

When I shoot large format architecture, I usually bring my Nikon F100
along and use that to determine my exposure.  Some guys are really
good and never need a meter and 
know the exposure at any given lighting situation. There are some
rules of thumb that correlate to the BDE. Open shade is 3
stops over BDE, overcast day is another 
factor of BDE, etc, etc.

John Luke