Is there a method that can determine if someone is using a proxy? How does youtube know....?
Well it was suggested, a long time ago, even before the view count "freeze" that YouTube were blocking a lot of proxy views.
It wouldn't take much to go around all the usual sources, collect the proxies, and put them in a black list.
I've even done it on a few sites I've worked on, so it's not rocket science.
Whether that's what they are doing, or not, you'd have to ask them

but it's certainly a very realistic suggestion that was thrown out there a while ago now.
Some people did report, early on, after the view "freeze" that they refreshed their browsers, and got a view, but when using a proxy, they didn't, so YouTube certainly seems to be using some anti-proxy measure.
It would also explain why some legit accounts seem to be complaining about some "legit" views missing as well.
If a friend surfed in on a proxy, and that proxy was on a black list, it wouldn't have counted.