There doesn’t seem to be an easy way to find out who is trusting you with hidden coordinates other than stumbling upon an obscured observation of theirs where you can see the true location or clicking through all of the pages of your Relationships tab on users settings - e.g. I have 25 pages of people to click through. There is also no notification generated when someone trusts you with their coordinates to bring awareness to this occurring.
It would be a nice tool for land managers, researchers, and anyone for whom true locations are helpful for their project to be able to see a list of people who trusted them with hidden coordinates. I could then, for example, target downloading just that narrower set of observations for my particular project.
It would prob go as an additional filter on the Relationships tab of user settings (e.g. “they trust you with their hidden coordinates: All/Yes/No”): https://www.inaturalist.org/users/edit#relationships