The category settled on no display
The glasses reaching volume are the ones without a screen. Road to VR's coverage of the Google and Samsung eyewear announcement notes that audio is the primary output for these devices, and that adding a display brings cost and interaction complexity that the mainstream products have chosen to avoid.
The reasoning is sound as product strategy. Displays are expensive, heavy and hard to make comfortable, and most of what people want from these devices arrives through a speaker. The side effect is that the device has almost no way to tell its wearer anything. A chime, a voice line, or a phone app in a pocket. That is the whole vocabulary.
An honest wearer still cannot answer the question
This is a different problem from the one raised by invisible controls, and worth keeping separate. Hidden input means a bystander cannot see an action being taken. This is about the wearer, who may not know either.
Somebody asks whether the glasses are recording. A wearer who wants to answer truthfully has to recall whether a session was started, trust that it ended when they think it did, or pull out a phone and check an app, which is slow enough that nobody does it. Assistant features complicate it further, since a device listening for a wake word occupies a state most wearers would struggle to describe. The result is a lot of sincere answers that happen to be wrong, and a bystander with no way to tell those from the other kind.
Build the check for the wearer first
We would require a one-second state check that works without a phone. Touch the frame and hear what the device is doing right now, in plain words, with no menu.
That fixes honesty and not verification, and we would rather say so than oversell it. A bystander gains nothing directly, because they still cannot audit anything and still have to take a wearer's word. What changes is that the word becomes worth something. Right now the most cooperative person in the encounter is guessing, and no amount of notice design further downstream repairs that.