They have such an indication already, an LED light on the other side of the frame.

Of course you have to be able to spot that. And trust that it really doesn't record when it's off (note that it simply may be covered by the user)

I seem to recall that when the snapchat glasses were a thing, they had a very bright an obvious ring of LEDs around the camera itself, that were bright enough to shine through a sticker placed over them. Sure, there are still ways to defeat that, but it makes it a bit harder.

Also I just googled for what the light actually looks like when it's recording, and it's not even really that visible...