How to call default node validation functions
If your form has a structure similar to the one used for the node edit form, then you can use node_validate(), which is called from the form validation handler for the node edit form.

function node_form_validate($form, &amp;$form_state) {
  node_validate($form_state['values'], $form);
}


node_validate() converts the first argument in an object using the following code.

  // Convert the node to an object, if necessary.
  $node = (object) $node;
  $type = node_get_types('type', $node);


Your form validation handler's code should be similar to the following one.

function mymodule_form_validate($form, &amp;$form_state) {
  // Run your validation code.
  node_validate($form_state['values'], $form);
}


As general rule, the validation code should go in the form validation handler. If the form validation handler set any error, the form submission handler is not called, and the user is returned to the form to correct what is wrong in the data that has been submitted.