As said from @Clive, the only way is to compare the value in $node with the value contained in $node->original. As far as I can see, that value is only available in hook_node_edit(), hook_node_update(), and hook_node_presave() since entity_load_unchanged() is called from node_save() (which also unset $node->original before exiting). If you need it in hook_node_submit() you could simply add the following code in your implementation.

if (!empty($node->nid) && !isset($node->original)) {
  $node->original = entity_load_unchanged('node', $node->nid);
}

Since you are checking for field properties, the other, more general, hook would be hook_field_attach_submit().

As said from @Clive, the only way is to compare the value in $node with the value contained in $node->original. 

As far as I can see, that value is only available in hook_node_update(), hook_node_presave() and every hook invoked from node_save() which receive the saved node as argument (but only when a node is updated), since entity_load_unchanged() is called from node_save() (which also unset $node->original before exiting). If you need it in hook_node_submit() you could simply add the following code in your implementation.

if (!empty($node->nid) && !isset($node->original)) {
  $node->original = entity_load_unchanged('node', $node->nid);
}

Since you are checking for field properties, the other, more general, hook would be hook_field_attach_submit(hook_field_attach_submit($entity_type, $entity, $form, &$form_state)). With this hook, $entity would contain the field value before the node has been edited, and $form_state contains the submitted values.