Mobile Hot Spot Help

I have a question on mobile hotspots. I have  an iPhone X with AT&T service, my plan is grandfathered and does not support using my device as a hot spot. I was recently in the caribbean  and the hotel wifi was lousy, so i rented a mobile hot spot. We used like four devices two of the devices had data. My question is how do we know whose data was being used when we were using the mobile hot spot? Thanks!