One of the things that I often say is: a tree must be healthy if you expect it to react to bonsai work in a predictable way. Assessing health in plants can be relatively straight forward in some cases, and more difficult in others. The more of a particular species you have the more comparison you can make, broadening your knowledge by illustrating the breadth of possibilities.
For Japanese Black Pine I find that spring growth elongation is one of the best indicators of overall plant health. There is a lot of nuance to be gleaned from the way that a tree sends out growth, particularly if you have dozens or even hundreds of examples to look through.
Seedling growth and top growth on young trees with sacrifice branches will often be the most vigorous. I remember seeing Jim Gremel walk into a BIB meeting one Tuesday years ago with what looked like a five-foot long walking stick covered in pine needles. He flopped it on the table and asked Boon if that’s what he meant by candle cutting. Black pine growth in the ground, where his tree was growing, can be exceptionally vigorous. In a container it will be slower but can still vary greatly among individual trees.
All the images in this post were taken on the same day, offering a glimpse into what it was like to walk around the garden examining different trees. The last two photos show a tree that has not had the resources to put on good growth. A season of sun, fertilizer and consistent watering will do a great deal to alleviate the problem.