When I say to "reasonably know", I don't mean everyone must be aware, but moreso that if you look around, not looking for cameras necessarily, you should notice it. The "reasonable person" standard is one that's commonly used in law, to describe the nature of something, even if the letter of it isn't necessarily true.

That said, assuming we're talking American law, this would all come down to case law anyways. A majority of American law isn't what's on the books, but what's worked out in court rooms across the country based on written legislation. Judges end up hashing out what the written law actually intends to mean (or in many cases what it should intend).

For my personal standards, I don't think even a sign is necessary. So long as it's in plain sight. Phone cameras are largely identifiable by the manner in which people hold their phones when recording others, so that would also be something I'd consider passing this "reasonable person" standard. Cameras built into pens and sunglasses though are very obviously intended to be concealed, and as such wouldn't without there being other ways to identify it; such as if it was told to those who'd be in range of the lens that they'd be recorded by this device.

There'd definitely be a lot of back and forth to hash out appropriate legislation, but I think it's very doable without significantly impacting the daily lives of people today.