I remember one of the Super 10/12/14/15/delete as applicable finals a few years back, that I watched on telly and you couldn't see a thing. Even the commentators were having a giggle about it. Apparently, pitch-side it was ok visibility, just anything from TV angle was a screen of pure grey.
I also vaguely remember a Quins game from about a decade ago (Newcastle at home), was a week night for some reason. I'd been working at the game up until kick-off. About two hours before a mist rolled in, and put the game in doubt. Again, right on pitch level you could see what was happening. More than two rows back in the stand - nothing. Rather than stay and watch the game, I just gave up about ten minutes in and went home.
Doesn't really help with the question I'm afraid. I would imagine that at the levels most of us referee, the worry about the fans back in the top of the stand, or Sky TV cameras not being able to see doesn't apply. So I too would go with "can I see the posts?".