When the deck goes to sleep, it does the suspend animation, so you know it’s going to sleep. If I remember right you’ll also have a notification in the bottom right of the screen saying low battery or something similar when it happens.

If the deck is asleep and gets low on power, nothing will happen. I guess if you wake up the deck with the power low enough it might flash the low battery notification and go back to sleep, but I’ve never had that happen. I had mine set to auto sleep at 5% and I never tried to wake the deck back up before I plugged it in after the auto suspend.

I was mainly using it back when there was a nasty bug that the deck dying from low power could result in the CPU/GPU being permanently throttled to 400 Mhz, effectively making the deck useless.