An RF power meter or spectrum analyzer. It would be very clear if a device is radiating enough RF energy to make a connection.
Battery life is another giveaway, I've had android phones turned off in a drawer for months and they still have nearly 100% charge when turned back on. If they were using enough power to make any kind of cell connection, the battery would be dead in a few days.
The link you have there even says "When your phone is turned off or runs out of battery, you won’t be able to track its real-time location"
And this just posted to HN today to explain in detail for you how the Iphone stays on and transmitting when you think it is off. https://arxiv.org/abs/2205.06114v1
Battery life is another giveaway, I've had android phones turned off in a drawer for months and they still have nearly 100% charge when turned back on. If they were using enough power to make any kind of cell connection, the battery would be dead in a few days.
The link you have there even says "When your phone is turned off or runs out of battery, you won’t be able to track its real-time location"