Here is a way to check if a Field in a feature class already exists or not. I was looking for an existing ArcGIS Esri function / tool but couldn’t find one so I wrote my own.

def FieldExist(featureclass, fieldname):
    fieldList = arcpy.ListFields(featureclass, fieldname)

    fieldCount = len(fieldList)

    if (fieldCount == 1):
        return True
    else:
        return False

This is a function and it assumes you have already set the workspace environment.

Usage:

        if (not FieldExist(myFeatureClass, "myField")):
          arcpy.AddError("Field 'myField' does not exist in " + myFeatureClass)
          sys.exit()

hope this is helpful.