Fields are attached to Entities, not Nodes. So the field is either attached to bundle that is your node (for all nodes, pre-existing or not), or it hasnt been attached to that bundle yet.
You can use field_info_field and look at the element bundles to see if your given field is attached to the given Entity. If it isn't you can use field_create_instance to add the field to your entity.
You can get a list of all entity types and inspect their bundle types with the bundles key returned from entity_get_info by calling $site_entity_list = entity_get_info(NULL).
Entities and fields are cached, so be sure you use something like drush cc all to be sure you're looking at up-to-date values for all these function calls.
EDIT:
You use these functions and PHP from essentially a small php script. Such business logic could be in a small module with hook_update_n() for instance. Or, you could use Drush and write a very simple PHP snippet using hte above 3 functions and some loops and call it using drush like:
drush php-script your-script-filename.php