That can be measured by frequency of code commits. You can tell which juniors are making the most of a rabbit hole and becoming better versus which ones are mindlessly surfing because they don't want to think. The first group will have things to share or talk about if they go some time without committing code and their mentor or manager starts getting worried.

If someone hasn't pushed something in a week or two, that's a good time for their mentor to initiate a conversation and see where they're at and when they think they will push something.

Anything else is micromanagement by a company that has hired someone without giving them any amount of runway to actually work and learn the space they're in. You can also tell on company hardware and on company accounts if the junior's browsing history represents getting better or just resting on their haunches because they want someone else to do the thinking. Let's not pretend the micromanagement cultures where "fear of rabbit holes" exists are not already doing this.