Do they deliver what is expected of them? What happens if you ask for more (as a test to see if they're slacking) over a period of weeks?
> Do they deliver what is expected of them?
If the answer to this question is “yes”, then congrats! You have a great team and shouldn’t waste your energy (and their well-being) worrying if they are working hard or not.
On the flip side, comparatively, it is much easier to compare performance vs other engineers working on the same project. At least for outliers.