Summary
When used with the Describe function, a Map Document returns a dataType of "MapDocument".
The MapDocument class from the arcpy.mapping module can be used to get additional information about a map document.
Map Document properties example (stand-alone script)
The following stand-alone script displays use of Describe with a .mxd
import arcpy
map_document = r"C:\Project\Project.mxd"
desc = arcpy.Describe(map_document)
print(desc.datatype) # MapDocument