Some historians appear to practice history professionally, in the sense that they produce histories that are accepted by other historians as "real" and "full." In this sense there is a profession of sorts, and exclusive body of persons who determine who may enter into appropriate practice.
However, there are many persons who lack this stamp of legitimacy who happen to speculate about, read about, or write about the past. Some appear to publish major works of non-fiction that HarperCollins might claim is a history. How can the difference between the amateur and the professional be detected when professionals also publish in HarperCollins (for example).
What are the criteria of professionality in history and which are fundamental?