Theming in Drupal 7 has always been a bit fiddly at the best of times. Unbeknown to me a small feature crept into a Drupal 7 release without me realising - the backport of theme debug settings from the much anticipated Drupal 8. This feature allows you to view debug information directly in the markup and not have to go chasing template suggesion names from within the Drupal admin screens.
In your Drupal 7 installation simply add this line to your settings.php:
$conf['theme_debug'] = TRUE;