have any of the podcasts ever delved into managing our own expectations of directs?
I've been having a hard time lately gauging the capacity of my directs ... the balance between not letting them settle, pushing them, but also not trying to force people to do more than they're capable of.
Managing Expectations
I'd suggest that your 1-2-1s will give you a good measure of the state of your DR's workload and mental capacity.
The true measure for me would be, are the completing what you are giving them? And is it having any adverse impacts on other elements of their productivity, or your team's completion of its goals? If No, then I'd suggest that they can manage it. After delegating new work just make sure they have time to settle in and get comfortable with the new responsibility.
Alternatively, you could tell your DRs to 'ask' if they have spare capacity, or if they feel they could accommodate some additional responsibility. Ambitious and aspiring DRs should be biting your hand off to show how much they can take on or demonstrate competencies across new areas. I myself am very open with my Manager if I have spare capacity or if I need something different/more challenging to work on.
I think it will be clear when someone hits a ceiling. Just keep an eye out for changes in behaviours, these could be warning signs.