A healthy forest is usually very noticeable. Most plants are green and growing.
Animals appear active and well.
Often there is an age structure, much like a human population, with both young and old present. There are multiple layers of tall, medium, and low vegetation, if not in one location, then certainly on a regional basis. Most importantly, reproduction is occurring, signifying the right amount of soils, minerals, nutrients, water, and other requirements for life. A healthy forest can almost be felt, as well as seen. In Hawaii, look at different forests, and see if you can sense the difference between a healthy forest, and one that is not.