Hi,
Given the improving quality of modern OE glass, and in a lot of cases, 3rd party glass, I believe that it has become almost impossible to tell the difference between either if the shot has been taken with the appropriate lens, is well exposed, and in sharp focus. Pixel peepers may disagree, but in the real world there isn't much discernible difference at all, if any.
However, once we start to discuss pro quality f2.8 - f4 super telephotos from Canon and Nikon, especially once you get past 300mm, then I personally believe that the prime will win every time. Fast telephotos need expensive glass, and it's this glass that will in most cases, if put under scrutiny, reveal that an f2.8 300mm prime will be superior to a f2.8 - f4 superzoom, etc, etc.
If correct technique has been used though, I still think that few could tell the difference.
It also takes an extremely good zoom to beat an 80mm prime for portrait or macro work, and I have yet to see a wide angle zoom that even starts to compare with a good wide angle prime.