Ask any domestic abuser. Most of them seem to be successful at it.
https://www.npr.org/sections/alltechconsidered/2014/09/15/34...
It’s crazy to me that people are being so skeptical of the idea. A lot of people share their logins freely with their spouses. I have never done it nor would I condone it, but it would be trivial for me to install spyware on the devices of many people I know, because they rightfully trust me. Not only do I know some of their device passwords¹, being “the computer guy” I could just outright ask for it or get them to input it anywhere while fixing some issue they have.
¹ And many more I have forgotten, because I make it a point to not record them, even mentally.
If you can get the app onto my phone in person, you can also just check which apps I have on my phone
That assumes continued access, which may not be true. Installing spyware gives you information down the line.
But if you have credentials and physical access you can just ask for their phone and straight up read their messages/apps.
Yeah, once, possibly under time pressure, and not at all times. Spyware gives you continued access.