how to read returned data format

how to read returned data format

Like on the following code:

      $('#<%= txtSearch.ClientID %>').autocomplete({
            source: function (request, response) {
                $.ajax({
                    type: "POST",
                    contentType: "application/json; charset=utf-8",
                    url: "WebService.asmx/GetAutoCompleteData",
                    data: "{'kwrds':'" + $("#txtSearch").val() + "'}",
                    dataType: "json",
                    success: function (data) {
                        response(data.d);
                        alert("message: " + data.toString());
                    },
                    error: function (result) {
                        alert(result);
                    }
                });
            }
        });
    }

Webmethod returns data, but I didn't know it returns the data in the following format d: ["first":"test", "last":test2]

My question is how to know, generally, the returned format of json data to avoid this confusion? Like d:[] or mydata:[] Or how to show the returned value through the web browser or any helping tools.