All Zuikos are good. I even have some older silvernoses which I have nothing to complain about There seem to be some rules of thumb to identify exceptionally good ones though.
I will try to compile all Zuiko lens-lore I could gather:
The 3.6/35-70 is said to be as good as primes and should be preferred to the 3.5-4.5 version.
The 1.8/50 lens, which has the 'made in Japan' inscription on the front ring, is said to be one of the best normal lenses ever.
Between the 1.4/50 lenses, the one with serials > 1 mio. should be preferred. I have nothing to complain about mine, which is in the 700k range, though.
All 2.0/n are said to be exceptionally good.
The 2.0/90 should be preferred over the 2.0/85 if the main subject isn't portrait, as it is said to be sharp as a razor-blade, which female subjects do not appreciate mostly.
There does not seem to be a general rule to identify single coated lenses. All what can be said seems to be:
All silvernoses are single coated
All non-silvernoses manufactured in the time of transition are marked MC if they are multi coated
All late lenses are multi coated even if they are not denoted as being such.
Unfortunately there is no way to identify a single coated lens if it is not a silvernose as Olympus later dropped the term MC