folderPath = 'C:\temp'
targetDataSource = 'C:\mapping\shapefile.shp'
files = os.listdir(folderPath)
for curFile in files:
if curFile.endswith(".txt"):
for lyr in arcpy.mapping.ListLayers(curFile):
if lyr.dataSource == targetDataSource:
print("Outdated layer found at " + lyr.lyr_file_path)