I have not addressed this matter in my previous post, I got caught up with the UT quotes so I will do it now.
I agree with your:
Quote:
'If it look like a Nazgul,
sounds like a Nazgul,
acts like a Nazgul,
chances are it's a Nazgul'
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philosophy, but as Rune well points out, it is not a single case.
However, as you already said, just like the race of Gothmog, there is no way to clearly tell.
As Legate showed, if Sauron wished to send one of the Nazgul as a messenger to Dol Guldur he could have.
But he didn't. Not at that time.
He sent messengers, emissaries and spies, that weren't able to bring him any tidings.
So only in late June 3018 did he eventually decide to use his best servants - the Ringwraiths which had until that point remained hidden.
The WK attacked Osgiliath, Khamul started to look for Gollum, and the story is well known from this point.