<?php
    include("conexao.php");
    $email = $_POST['email'];
    $sql = "SELECT * FROM usuarios where email='".$email."';
    $res = mysql_query($sql);
    $row = mysql_num_rows($res);
      if($row > 0){
        echo "1"; // retorna um error
       }
     else{
       echo "0"; // retorna tudo ok
    }
?>