You can set your Echo and Google Home or Nest devices to give you audio cues when they start—and, more importantly, stop—listening for commands.
Google Assistant commands you must know - Android Authority
How to stop Alexa, Google, and Siri from listening to everything you say
Smart speaker recordings reviewed by humans
Alexa, how much does know about me?, Technology News
How to make Alexa and Google Assistant let you know when they start—and stop —listening
How to stop Alexa, Google, and Siri from listening to everything you say
How to make Google Home & Echo talk to each other in an infinite loop
Everything you need to know about the Echo - CNET
Building an Intelligent Voice Assistant from scratch, by Marcelo Rovai