You can always tell. Look back. That’s all you have to do. Sometimes you can sense that awkward “should I reach out for a hug?” moments that longer until you either go for it or let the moment pass you by. But you can’t rely on that. You look back after the goodbye. When she’s walking away. When he’s about to close the door. Did she turn to peek at you one last time? Did he turn around to snag another look? If they do, they’re interested.