# Import modules
import arcpy
import sys
import traceback

    # Set local variables
    indata = arcpy.GetParameterAsText(0)
    fieldName = "PrintKey_GIS"

    try:
       # check if the PrintKey field exists
       if arcpy.ListFields(indata, fieldName): #if field exists, evaluates to true
           arcpy.AddMessage("PrintKey_GIS field exists")

       else:
           # Add a new field of that name
           arcpy.AddField_management(indata, fieldName, "TEXT", "", "",25, "PrintKey_GIS" , "NULLABLE", "", "",)
           arcpy.AddMessage("PrintKey_GIS field does not exist")

    except Exception as e:
        arcpy.AddMessage(arcpy.GetMessages())

print "Adding new field first time"
arcpy.AddField_management(fc, "NewField", "TEXT")
print "Adding new field second time"
arcpy.AddField_management(fc, "NewField", "TEXT")
print "Adding new field third time"
arcpy.AddField_management(fc, "NewField", "TEXT")

Adding new field first time
Adding new field second time
Adding new field third time

Executing (Add Field): AddField C:Users<username>DocumentsArcGISDefault.gdbfc NewField TEXT # # # # NULLABLE NON_REQUIRED #
Start Time: Tue Aug 21 11:01:38 2018
WARNING 000012: NewField already exists
Succeeded at Tue Aug 21 11:01:38 2018 (Elapsed Time: 0.08 seconds)
Executing (Calculate Field): CalculateField C:Users<username>DocumentsArcGISDefault.gdbfc NewField 'blah' PYTHON #
Start Time: Tue Aug 21 11:01:38 2018
Succeeded at Tue Aug 21 11:01:38 2018 (Elapsed Time: 0.12 seconds)

if len(arcpy.ListFields(fc, fieldName)):
    #  Field exists.  Do something.
else:
    #  Field does not exist.  Do something else.

import arcpy
import os

arcpy.env.workspace = r"C:UsersSanjayDocumentsArcGISSimilarPoints by attribute in polygonmygdb.gdb" #Parameter1

def FieldExist(featureclass, fieldname):                   #* Start if function to check if field exist  */
    fieldList = arcpy.ListFields(featureclass, fieldname)

    fieldCount = len(fieldList)

    if (fieldCount == 1):
        return True
    else:
        return False                                    #* End of function to check if field exist  */

datasets = arcpy.ListDatasets(feature_type='feature')
datasets = [''] + datasets if datasets is not None else []

ListOfFieldNeeded = ["ID","Creator","checker","Approver"]#parameter2  #List of field to check if not exist add it.
for Afield in ListOfFieldNeeded:                           #will check each fc in gdb and add field
    for ds in datasets:
        for fc in arcpy.ListFeatureClasses(feature_dataset=ds):
            if not FieldExist(fc,Afield):
                #print "{} Not Exist in featureclass {}".format(Afield,fc)
                arcpy.AddField_management(fc,Afield,"TEXT")
                print "Field {} added in fc {}".format(Afield,fc)   #Addmessage to print model builder environment
                #you can add your script validation using else or exception occures