Identifying Importer Posts

Two things I could imagine:

• Check the post_modified value. Maybe import creates a definitive timestamp that you could use. You’ll still have to save the import date somewhere so you can check against it.
• I do some post importing via a stream/HTTP response (this is not the native importer). During the import, I map my SysBot Plugin user to every post that has no existing user in the installation. This allows me to use archives, filter the admin post list table, etc. A very convenient solution to check those. It also allows posts that get reviewed to get out of the “imported but not touched yet”-queue as the author can change. You might also be able to use the native importer and use the 'import_allow_create_users' with simply setting a callback of __return_false.
• Infiltrate the WP importer and attach a meta value during import. There’s a hook that triggers for every imported post meta key named 'import_post_meta'.

In case you’re infiltrating the native importer plugin, you’d need to start at the 'import_start' hook. The last hook in the system (where you might want to check if everything went fine), is the 'import_end' hook.

EDIT

I just encountered a class that I’ve not even known that it exists (in core, not the WP Importer plugin): WP_Importer. This class has a method, named get_imported_posts().

So in theory you could do the following:

$importer = new WP_Importer;
$importer_name="???";
$bid = get_current_blog_id();
$imported_posts = $importer->get_imported_posts( $importer_name, $bid );

The meta key that gets searched for is built by the two meta keys:

$meta_key = $importer_name . '_' . $bid . '_permalink';

So there seems to be a unique trace route that one can follow.

Update #2

An example query you could run would be:

var_dump( $wpdb->get_results( "
    SELECT post_id, meta_value 
    FROM {$wpdb->postmeta} 
    WHERE meta_key 
    LIKE '%_permalink%'
" ) );

This should bring all similar posts up an allow you to determine the “importer_name” easier.

Edit #3

There’s get_importers(), which you can dump to see what importers are registered (using register_importer()). This should help identifying the actual importer name.