That's a failure of upper management to demand accountability from him. A lot of people hide a lot of stuff even when they are at the office.
> What interview questions can you ask to very "dedicated remote workers" from "easy paycheck remote workers"?
The same you ask to discern between "dedicated office worker" and "minesweeper-champion office worker"?
> How to you improve performance in remote workers
Promote accountability based on deliverables and targets. Establish always-on communication channels and systems, keeping remotes involved in the decision-making rather than being recipients of orders. Have periodic reviews, particularly if things are slower than expected. And at the end of the day, don't be scared to let people go if they are not meeting expectations, or to put your foot down on things like webcam usage if you really need it.