How to check my Mac mini M1 is really sleeping?

[Basic75]

That's not a reliable test with Macs. Especially if Power Nap is enabled, the computer will respond to a ping even when asleep.

I don't think so, Power Nap (wake occasionally to fetch new mail and do other housekeeping tasks) is not Wake On Lan (recognise special network packets that wake the computer from sleep mode). Unless you mean that with Power Nap it will occasionally respond to a ping because it took a short break from sleeping.

 

[Basic75]

How can I check that my Mac Mini M1 is really sleeping/in standby.

Attach a USB mouse with a visible light that turns off when the computer is sleeping. That's my method for confirming sleep now that the pulsating power light is no more.

It works well for me, although occasionally the mouse won't light up after waking, though I'm assuming that to be due to it being connected to a USB hub instead of directly.

 

[Basic75]

So I think I will do a complete new fresh re-install of macOS.

Before you take drastic measures, open Terminal.app and enter

pmset -g

to see whether some unexpected applications are preventing the computer from sleeping. This has become worse over time.

It's especially bad on a laptop when you need it in the morning and find out the battery is flat because some ******** process kept the machine awake all night.