I still recall that conversation many years ago, my fellow point at a #CathayPacific flight which had just launched into the sky, telling me that it was a cargo flight. It was quite far away and I was wondering how he could have distinguished that easily. He said, simple but small details, cargo flights had very few (if any) passenger windows.

(Source: My Freighter)
https://www.aircargonews.net/wp-content/uploads/2022/10/JM279050.jpeg