Dennis,
Here is my solution, see attached.
I have one of these home-made grey cards in every lens cap. They are printed on good photo paper with colour management . The grey's will not be perfect of course but it seems to work in practice.
You can now buy a fancy little cube device with greys etc. on it Also there's a thing called a colour passport made by X-Rite that has numerous grey & colour patches on it. You could get paranoid using that  

Edit: just checked the attached image on my screen with a colour picker and it's pretty well spot on 
Terry

Me too except I do set the camera to the most appropriate value for the prevailing conditions. Some camera enable you to bracket WB too.
When using the Control key to highlight possible areas for WB correction, surprisingly small areas can sometimes be selected but as a last resort I've used a WB filter and manually tweaked the R & B values there. Not ideal but better than nothing.
I seem to remember Mike saying something about this idea in the early days. To do it properly, a profile of every camera is required, ok. for the big boys, difficult for Mike. If you compare colour temperatures reported by several different raw programs, I've found they nearly all give a different value for the same camera & setting and visual setting is not accurate.
Terry.