Do these devices record all the time or only after the trigger word (they would need to be always listening for the trigger word) until the end of the statement?
It's got a small couple-second buffer (enough to store "Amazon" or "Computer" or "Alexa" or "Echo") where it takes what it hears and compares it with its internal model for a match.
If there's no match, the buffer is overwritten with the next bit of noise. Once the device gets a wake word match, it transmits the statement that follows to home base to transcribe and handle.
In practice we know that trigger words for all these devices occasionally misfire (There was also an issue with one type of google home device a while ago which was shipped with a faulty physical button that caused it to be turned on at intervals as if the user had pressed the physical button to start speaking).
And would we know if they had been recording unnecessarily?
And you can look at network traffic (e.g. from wifi router stats) to be pretty confident they're not constantly live-streaming audio up to the cloud.
Of course most people will not actually do this monitoring themselves, but there are enough of these devices out there that if a significant number started recording constantly somebody would notice pretty quickly. And that would be terrible PR for the company involved, so I think google and amazon and apple have a pretty strong incentive not to do this.
The PR angle isn't that reassuring to me either, they've already absorbed some pretty bad PR hits on these devices and they're still going strong.
a) viewing what they store via their log tools (though this isn't guaranteed to show everything, ie if they are recording everything they couldhide)
b) monitoring outbound network connections