Is a node linked in any Panel (Drupal 7)?
You are looking for something along the lines of file_usage_list() I guess. I also assume that you are interested in the "Add content > Existing node" functionality of Panels.

You could implement your own function, with the following code:

function _panels_node_usage_list() {
  $result = db_query(
    "SELECT * FROM {panels_pane} WHERE type = 'node' AND subtype = 'node'"
  );
  $node_usage_list = array();
  foreach ($result as $pane) {
    $pane_configuration = unserialize($pane-&gt;configuration);
    $node_usage_list[$pane_configuration['nid']]++;
  }
  return $node_usage_list;
}


and then call this function from your own implementation of hook_node_access() where you can check if the nid of the node to be deleted is in the array returned by _panels_node_usage_list().

Good luck!