[Cub3]

> build the relationship enough for them to open up to you

Completely agree, it's also about building that open and honest culture to enable having that kind of conversation with the entire team. But even with this it still doesn't catch many day-to-day frustrations or things you may pick up on that the person wouldn't offer up regardless.

> having an always-on call

The problem with this is you put the onus on the Junior to ask the question, which is hard for someone who's still learning (especially with imposter syndrome being so rife). Also this doesn't allow for the Senior to spontaneously create learning opportunities relevant to whatever they're currently doing, the barrier cuts down a lot of it.