What method should an ArcGIS Desktop user use to find out the data type referenced by a layer?

To determine the data type referenced by a layer in ArcGIS Desktop, accessing the layer properties and looking at the Source tab is the appropriate method. The Source tab provides detailed information about the data source for the layer, including its type (e.g., feature class, raster dataset, etc.) and location. This information is crucial for users to understand the nature of the data they are working with, which can influence analysis and map creation.

The Source tab will also display additional details like the data's path and whether it is stored locally or in a geodatabase, enabling users to make informed decisions about data management and processing. Thus, selecting the Source tab is the most direct and informative way to ascertain the data type associated with a layer in ArcGIS.