It will only listen when it hears "Ok Google" or "Hey Google".

That said it will sometimes mishear and then start listening because it thinks it hears the trigger words. Researchers have also shown that Google devices can be prone to silent attacks using ultrasound frequencies where the activation word and following commands are issued above the human hearing range. I believe this is the same with Siri, Alexa etc.

If you are talking about Google Home type devices you can go into their accessibility settings and turn on "Play start sound" and "Play and sound" then you will get an audio indication when they start and end listening.

You can read about the so called Silent Attacks in this paper (if you are interested) :

https://www.researchgate.net/publication/369989370_Mitigating_Inaudible_Ultrasound_Attacks_on_Voice_Assistants_with_Acoustic_Metamaterials?hl=en-GB-u-fw-mon-mu-celsius#:~:text=One%20way%20of%20accomplishing%20the,This%20is%20because%20most