Variable-opacity headphones

The actual speakers in the headphones are small earbuds or similar, and that's surrounded by a large liquid crystal device the size of large headphones that goes transparent to indicate that they're not in use.

The user can set the device to lie for privacy or whatever, but in normal use people will know whether you'll hear if they talk to you by whether you look like you're wearing large ear covers or not. They can be seen more easily and from furthur away than the small earbuds.