I'm trying to fill IRS forms (for example https://docs.google.com/document/d/1ZpqwUusHIOKToKhf8eufywS_8lGxcDuGo0AcNGsQmyw/edit#) programmatically using ITextSharp:

var file = @"w8BENEV1021.pdf";
        var fileS = @"w8BENEV1021_1.pdf";
        using (var reader = new PdfReader(file))
        {
            reader.RemoveUsageRights(); // This is needed for some readers (acrobat) so they will not show pop that we sealed the file to changes

            using (var memStream = new MemoryStream())
            {
                using (var stamper = new PdfStamper(reader, memStream))
                {
                    AcroFields fields = stamper.AcroFields;

                    fields.SetField("topmostSubform[0].Page1[0].f1_1[0]", "nameodfodod");
                    fields.SetField("topmostSubform[0].Page1[0].c1_1[12]", "13");
                    fields.SetField("topmostSubform[0].Page2[0].c2_5[0]", "11");

                    stamper.FormFlattening = true;
                    stamper.Close();

                    var data = memStream.ToArray();
                    File.WriteAllBytes(fileS, data);
                }
            }
        }

I was wondering how do I know to which control in the PDF does each key maps to (like topmostSubform[0].Page1[0].f1_1[0]) is there some GUI that shows that?