<form class="form-horizontal" action="cadastrar.php" method="GET">
                <fieldset>
                    <legend>DADOS CADASTRAIS</legend>

                    <input type="hidden" required id="cep" name="cep" value="<?php echo $cep; ?>">

                    <div class="row">
                        <div class="col-md-3">
                            <label for="cpf">CPF</label>
                            <input type="text"
                            class="form-control"
                            required
                            id="cpf"
                            name="cpf"
                            placeholder="SOMENTE NUMEROS">
                        </div>
                    </div>
                    <div class="row">
                        <div class="col-md-12">
                            <label for="nome">NOME</label>
                            <input type="text"
                            style="text-transform:uppercase"
                            class="form-control"
                            required id="nome"
                            name="nome"
                            placeholder="NOME DO CLIENTE"
                            required>
                        </div>
                    </div>
                    <div class="row">
                        <div class="col-md-4">
                            <label for="end" >ENDERECO</label>
                            <input type="text"
                            style="text-transform:uppercase"
                            class="form-control"
                            required
                            id="end"
                            name="end"
                            placeholder="DIGITE ENDERECO">
                        </div>
                        <div class="col-md-2">
                            <label for="num">NUMERO</label>
                            <input type="text"
                            class="form-control"
                            required
                            id="num"
                            name="num"
                            placeholder="NUMERO">
                        </div>
                        <div class="col-md-2">
                            <label for="comp">COMPLEMENTO</label>
                            <input type="text"
                            class="form-control"
                            id="comp"
                            name="comp"
                            placeholder="COMPLEMENTO">
                        </div>
                        <div class="col-md-4">
                            <label for="bairro">BAIRRO</label>
                            <input type="text"
                            style="text-transform:uppercase"
                            class="form-control"
                            required
                            id="bairro"
                            name="bairro"
                            placeholder="BAIRRO DO CLIENTE">
                        </div>
                    </div>
                    <div class="row">
                        <div class="col-md-6">
                            <label for="cidade">CIDADE</label>
                            <input type="text"
                            style="text-transform:uppercase"
                            class="form-control"
                            required
                            id="cidade"
                            name="cidade"
                            placeholder="CIDADE DO CLIENTE">
                        </div>
                        <div class="col-md-6">
                            <label for="telefone">TELEFONE</label>
                            <input type="text"
                            class="form-control"
                            required
                            id="telefone"
                            name="telefone"
                            placeholder="TELEFONE">
                        </div>
                    </div>
                    <div class="row">
                        <div class="col-md-6">
                            <label for="rg">RG</label>
                            <input type="text"
                            class="form-control"
                            required
                            id="rg"
                            name="rg"
                            placeholder="SOMENTE NUMEROS">
                        </div>
                        <div class="col-md-6">
                            <label for="email">E-MAIL</label>
                            <input type="text"
                            style="text-transform:uppercase"
                            class="form-control"
                            required
                            id="email" n
                            ame="email"
                            placeholder="EMAIL DO CLIENTE">
                        </div>
                    </div>
                    <div class="row">
                        <div class="col-md-12">
                            <br>
                            <button type="submit" class="btn btn-primary">CADASTRAR</button>
                        </div>
                    </div>
                </fieldset>
                <!--FINAL FORMULARIO DE CADASTRO-->
            </form>

http://localhost:8080/sys/functions/functionCPF.php?cpf=11122233344

{"dados":{"cpf":"11122233344"}}

{"dados":{"cpf":0}}

$(document).keydown(function(){
    var cpf = $("#cpf");
        cpf.blur(function() {
            $.ajax({
                url: '../functions/functionCPF.php',
                type: 'GET',
                data:{"cpf" : cpf.val()},
                success: function(data) {
                console.log(data);
                data = $.parseJSON(data);
                $("#resposta").text(data.cpf);
            }
        });
    });


    });

<?php
//database configuration
    $dbHost     = '127.0.0.1';
    $dbUsername = 'root';
    $dbPassword = '';
    $dbName     = 'master';
#Verifica se tem um cpf para pesquisa
if(isset($_GET['cpf'])){
    #Recebe o cpf Postado
    $VarCPF = $_GET['cpf'];
    #Conecta banco de dados
    $Mysqli = new mysqli($dbHost,$dbUsername,$dbPassword,$dbName);
    $sql = mysqli_query($Mysqli, "SELECT
    cli_cpf
        FROM
         big_clientes
         WHERE
             cli_cpf = '".$VarCPF."'") or print mysql_error();
    #Se o retorno for maior do que zero, diz que já existe um.
    if(mysqli_num_rows($sql)>0)
        echo json_encode(array('cpf' => 'CPF JÁ CADASTRADO!'));
    else
        echo json_encode(array('cpf' => 'CPF OK!' ));
}