Where a leaf or soil analysis is not helpful (e.g. the lab has no
reference for a particular plant's NPK etc. requirements due to the
plant's being uncommon), and one has to fall back on visual cues to
find out what is missing from a plant's nutritional intake (assuming
otherwise balanced nutrition), how does one visually distinguish
Magnesium from Potassium deficiencies?

Both affect older leaves. Both cause marked interveinal chloriosis
with reduced new growth. I know that in some cases, Potassium
deficiencies cause corrugation deformities in new growth as well (e.g.
passionfruit), but not always. I also read that Potassium shows marked
scorching on the leaf edges, but again, not always, and indeed,
pictures of Magnesium deficiencies in some crops also show necroses in
affected leaves. The one symptom (and its a judgement call rather than
an absolute, which isn't really helpful) that Potassium has in
contrast to Magnesium deficiency, is that affected leaves are dropped
more reluctantly with the former than in the latter.

Can the readers of this forum chime in on what can be used as
distinguishing criteria between the two deficiencies? I'm fairly sure
that Nitrogen deficiency isn't a factor, because with that deficiency
I believe that there is an overall yellowing, and interveinal chloriosis
is not generally noted.

Thanks in advance.