Are there diagnostic tests to distinguish between proper and common nouns?
Are there some fill-in-the-blank type questions that, if one were to fill in the blank and it sounded right to a fluent speaker then it would have to be a proper noun (or it would have to be a common noun)?
Solution 1:
It's rare, though not impossible, to have an article in front of a proper noun, e.g. "The Alan...", "A Steve...". But these are not always completely bad: "The Alan with the white shirt...", etc.
Still, even in the latter example I think it sounds awkward enough to provide a decent heuristic, if that's what you are looking for.