I've got a late 2015 iMac with a Bluetooth keyboard and mouse. As part of the minimalist design, Apple appears to have eliminated all the indicator lights. If the screen is black and the computer doesn't respond to either the keyboard or the mouse, how can I tell (a) a computer that's been turned off from (b) a computer where the batteries on the keyboard and mouse have run down?
If the computer is off, I don't want to turn it on, so pressing the power switch is out.
(This question came up in the context of unplugging the computer from a malfunctioning UPS, where I couldn't tell if the UPS had shut the computer off or not. If the computer was off, I didn't want to turn it on and risk having the UPS shut it off again halfway through the startup routine; if it was on, I wanted to shut it down properly before unplugging it.)