The tribe_event_is_all_day() function is exactly what I would have suggested. Could you perhaps share your complete code via Pastebin, Gist or some similar service so I can see what might be going wrong (can you also indicate where it is being used, exactly)?
You will see the code on line 62-66. The file is the List Widget for the Pro version (override in /wp-content/themes/theme-name/tribe-events/pro/widgets/list-widget.php). Im trying to write a custom message if the event is an All Day Event.