arcpy: checks if ArcMap layer is a CAD layer

isCADLayer.py

def isCADLayer(layer):
    ''' Returns True if a layer is a CAD layer.
    '''
    desc = arcpy.Describe(layer)
    if hasattr(desc, "dataType") and (desc.dataType == "FeatureLayer"):
        describeTable = arcpy.Describe(desc.catalogPath)
        if describeTable.dataElementType == "DEFeatureClass":
            describePath = arcpy.Describe(describeTable.path)
            if hasattr(describePath, "dataType")
            and (describePath.dataType == "CadDrawingDataset"):
                return True
    return False


import arcpy
mxd = arcpy.mapping.MapDocument("CURRENT")
lyr = arcpy.mapping.ListLayers(mxd)[0]
print isCadLayer(lyr)