Set the label expression and switch on labels for all layers in an ArcMap document.

toggleLabelsInMXD.py

#
# @date 17/09/2014
# @author Cindy Williams
#
# Set the label field and switch labels on for
# all layers in a mxd.
#
# For use in the Python window in ArcMap.
#

import arcpy

mxd = arcpy.mapping.MapDocument("CURRENT")

for lyr in arcpy.mapping.ListLayers(mxd):
    if lyr.isFeatureLayer:
        f = arcpy.ListFields(lyr, "*GIS*")
        if f:
            lyr.labelClasses[0].expression = "[" + f[0].name + "]"
            lyr.showLabels = True
        else:
            print lyr.name
    lyr.visible = True
    
arcpy.RefreshActiveView()