I use
$.post('ajax/test.php', function(data) {
$('.result').html(data);});
to send data and retrieve information and show it to the user. But “hackers” can access my file (test.php) just typing it in URL. Is it possible to detect if the call is made from jQuery or not?
Yes, but the same “hackers” can as easily fake a jQuery call. There is no reliable way to detect whether a call was made from jQuery, or using other means. Anybody can make a request to a resource.
If you have sensitive data on the web, you need to protect it using classical means like a user login, HTTP basic auth, or IP limitations – just like a normal web page.