var isJson = true;
try {
    JSON.parse(data);
} catch(e) {
    isJson = false;
}

$.ajax({
    type: "POST",
    url: "http://******",
    data: {*****},
    headers: {*****},
    success: function (dt, status, request) {
        var isJson = request.getResponseHeader('Content-Type') == 'application/json';
    }
});