If it was virtually 100% accurate when the vector was overlaid on the drawing then it probably was a trace. Anyone that is skilled enough to do that would put it in their own style instead of copying the shows. Like I said though, virtually. There is a chance that it wasn't traced, but I'd place my bets that it was.
Of course for those special people with a good memory. Its pretty much possible.
Here is stephen wiltshire drawin rome. Its been awhile, but I remeber seeing this guy on yahoo pages. [link]
Thats pretty much down the millimeter without a tool or ruler.
There is also the generas of photorealism, or hyperealism. Search via google. There is also matt painter who do a good job too.
But as for perfection, most of the artist pretty much are trickers of the eye. We could paint many strokes, and tediously it can look accurate from far away. This is a good reason why you should go to musuems, because looking at a big painting from both far away and then studying it up close make a huge difference. From far away, it can look realistic but up close its just alot of various painting strokes.
Reminds me of some really great digital works I've looked at on dA for the purpose of judging them to see if they're something my group (mentioned in my sig) is looking for. Sometimes, I see some REALLY sloppy craftsmanship that's only visible if you look at it in its largest resolution.
And even if it is possible, that's hardly a talent worth developing. A real artist needs to be able to reduce 3 dimensions down to 2 for most forms of visual art. (Sculpture being an obvious exception) And the time spent getting that accurate reproducing a photo or other 2d image is time that could have been spent on actual useful things.
Really the only way you can confirm a trace is to get the source and sit it right next to the artwork.
I don't feel there is any need to be technical with (be it using a grid, copypasting, or eyeballing) a trace when it's obvious that the artist did not create any conceptual part of the piece themselves, be it the pose, character, or the image itself.
If they were doing it from a picture they had on a screen right in front of them, I'd say it's possible. Although I also don't think someone with that sort of skill would use it to make perfect replicas of cartoon ponies. And it still stretches credibility that they'd get everything down to the 'millimeter'. It's even more possible that they used a grid of some sort.