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 mark-up and not have to go chasing template suggestion names from within the Drupal admin screens.
In your Drupal 7 installation simply add this line to your settings.php:
$conf['theme_debug'] = TRUE;