Well, it's been a long time since sophomore physics, so my optics
knowledge is......rusty.

Is there any way to tell from the product descriptions how close the
work needs to be to be in focus?

For what I need, I think anything from about 6" to 12" would be OK. I
would also want an inch or two of latitude where it would stay in focus.

Speaking as a general rule on inexpensive magnifiers, no there is no way
to tell from an Internet product description what the focal length is,
nor the depth of the field for the focus range. Other than the higher
the magnification, the closer the focal point.

IIRC, makers like Carl Ziess don't like to use the term 'magnifiers' for
the 6 to 12 inches range. They tend to be more 'binocular' that is two
small telescopic like devices attached (protruding) from a pair of
glasses. The longer the focal distance (and the wider the focal range)
requires many lenses.

some reading material:
http://www.opticsplanet.net/how-to-c...agnifiers.html

Try before you buy.