Detecting Join programmatically using ArcPy?

Too bad there's not a hasJoin property on the arcpy.Layer class. I think you can test for a join by looking at field names though. Here's a simple proof of concept for data in a file geodatabase:

import arcpy, arcpy.mapping as arc

def joinCheck(lyr):
  fList = arcpy.Describe(lyr).fields
  for f in fList:
    if f.name.find(lyr.datasetName) > -1:
      return True
  return False

arcpy.env.workspace = r'<path_to_your_gdb>'
mxd = arc.MapDocument(r'<path_to_your_mxd>')
lyrs = arc.ListLayers(mxd)
for lyr in lyrs:
  # ignore group layers
  if not lyr.isGroupLayer:
    hasJoin = joinCheck(lyr)
    if hasJoin:
      print '\nFound a join: %s.' % lyr.datasetName
    else:
      print '\nNo join found on %s.' % lyr.datasetName