If your featureclasses are in Feature Datasets, then [@Aragon's solution][1] will not work because you need to walk through the feature datasets and then check the featureclasses.

Using [@Aragon's code][1] as well as @gotchula's code from [List all feature classes in GDB, including within feature datasets][2], I produced the following code that will read through all feature datasets and feature classes in a geodatabase.

**NOTE**: By default ListFields appears to be case-sensitive.  I have added code to convert both the user-defined fieldname and the ListField name to lowercase for case-insensitive searches.  You probably would also want to disable most of the print statements, but I left them in there so you can follow the code as it is running.  I also created the sub-function FindField within the so that I could call it multiple times in the script without having to re-code it.


    import arcpy
    
    def FindField(fc,myField):
        fieldList = arcpy.ListFields(fc)
        for field in fieldList:
            if str.lower(str(field.name)) == str.lower(myField):
                print "    " + fc + " contains fieldname: " + myField

    myField = "test"
    arcpy.env.workspace = "D:/test/data.gdb"
    
    #Search root level featureclasses
    for fc in arcpy.ListFeatureClasses():
        print "Searching root level Featureclasses..."
        print "  Searching " + fc
        FindField(fc,myField)
    
    #Search Feature Datasets
    for fds in arcpy.ListDatasets('','feature'):
        print "Searching FeatureDataset: " + fds
    
        for fc in arcpy.ListFeatureClasses('','',fds):
            print "  Searching Featureclass... " + fc
            FindField(fc,myField)


  [1]: https://gis.stackexchange.com/a/26898/115
  [2]: https://gis.stackexchange.com/questions/5893/list-all-feature-classes-in-gdb-including-within-feature-datasets