White balance with JPG

Archived from groups: rec.photo.digital (More info?)

I know the value of RAW and how much easier is white balancing with
RAW. I have experience having done that.

If, however, what I have to work with is a bunch of JPGs, are there tips
to obtaining a nice balance above and beyond just eyeballing it?

Specifically, with a stream of images illuminated by evening sunlight as
well as an external flash, the two differing colors in light are giving
me a very difficult time finding the balance. (Tried levels, curves,
color balance and had very similar outputs) These images do not have
white objects or black objects (beyond distant shadows inside tree
leaves). I might find a gray tone but only by way of a shadow cast onto
the skin of a subject.

Even if I had a RAW version of these and still had no white/gray objects
for basis (which I'm accustomed to) what, quantitative techniques would
make it easier?

If the photo session began with a white object, or a gray card, could I
use this as a benchmark and then slide each image under my adjustment
layers for the "correct" look?