Hi--

>What I am really hoping to find is a simple and reliable test for lens
>optical quality that does not require special tools, etc. and that could
>be applied "in the field", for instance when one is offered a lens for
>sale at a camera show. I suppose this must be the holy grail of lens
>buyers.
>
>If you know of such a test, kindly let me know. Thanks in advance.

Ah, the simple, all-revealing, filmless instant test, huh...? ;-)
The best I have come up with (F, F2, *F3* finders only...), is to
check for correct infinity focus at opposite edges, then corners
(can show non-parallelism, which can indicate centering problems).
On film, shooting two infinity-focus verticals without changing
focus (at the widest aperture at which good performance can be
expected...), the first with the shutter release end up, the
other with the camera inverted, can show the presence of
differences across the frame line in the two otherwise
identical frames...