Ho to check if php session is valid?

Ho to check if php session is valid?

I'm using Zend framework 1.12 and I've been built a web app where the
users has to be logging in order to do tasks.
All the task are made through json calls to functions in php modules.
The problem it's when the session has expired and the user wants to
execute a task, the response from the JSON it's the login page and the
response code it's 200.
There is a way to check if the session it's expired?
In the controller of user validation I've:
if ($User->isValid()) {
$this->_redirect(base64_decode($this->redirect));
} else {
return $this->render('logging');
}
Can you help me?
Thanks