There are a bunch of ways to go about it.
Some, for example, would even say something like font colour. If you're reading a text and you see it's in a certain colour, you'll say, “Oh, that's AI.” It's the same thing with voice over the radio. If you hear a certain tone in the background, you think, “Oh, that's AI.”
There are a thousand different technical ways to go about it, but it would have to be something that would make a user think instinctively, as soon as they saw it, “Oh, yes—this is AI,” and have that be a global standard.
There is the logo of the little four-legged star. You'll see that around in various things. It's a step in the right direction, but we're at the very beginning of a very big, complex problem. It will take clear direction from governments around the world, basically, that this is required and that we expect this of companies that are building AI.