The "Event Log" on a device page, DOES tell you if Alexa was used to turn on a device.
Same as I told you here: Amazon Echo Skill Logging - #10 by jtp10181
It does not go to the system log, you will only see it in the event log for a single device at a time.
If you want a history of voice commands used on Alexa, you can get that on your amazon account somewhere.
