On our Professional mat controller, how can I tell if the timer’s set time has elapsed and the mat has powered off?

Ereada mats are professional devices, and the controllers are some industrial-grade. They do not show current or set temperature (only level) or time remaining. 

When the timer indicator goes off, it means that the time elapsed.

But really, you can buy an additional digital plug-in timer and connect the mat controller to the socket through it. It will help you control time better and be double safe if you use your mat at night.