As we've seen time and again, "expert" is in the mind of the beholder.
But I will say that most good instructors can spot the technical strengths and deficiencies in high level skiers even on green groomed terrain. It's not only about the amount of skill a skier has, it's also a question of how those skills are applied. In other words, there are good and bad movements, and good and bad tactics, and you can become highly skilled at any of them!
Nearly any technique and tactic works on groomed green terrain. It's not very hard to get the skis into a skid, and it's not hard to control the skid, once you've gotten them there. So many skiers feel like heros on green groomed terrain. But a good eye will easily see that these "heros" are really only hacks highly skilled at bad skiing!
The farther you get from perfect groomed corduroy, the more important it becomes to use good technique--toward either extreme. Ice, on one end of the spectrum, makes it very easy to get the skis into a skid, but very difficult to control that skid once you're there. At the other extreme, deep, heavy snow and crud make it virtually impossible to push the skis into a skid at all. And if it's inconsistent, it's also very hard to manage a skid in the stuff.
But again, trouble in these extremes doesn't necessarily mean a lack of skill. It often reveals the "pseudo expert" who is highly skilled at ineffective techniques. And this poor technique would easily show up on the green groomers too, if you know what to look for.