1. For me, a stressed tree is a tree that is not sure what it should do. It puts survival as a top priority, instead of equalized growth output it looks like it's making random decisions. It's making more hormones than it regularly should make because there's something wrong, or it's producing molecules that serve a protective function even though the conditions don't require those kind of protections.
2. Infections, water issues, pH issues, drainage issues, chemicals, human interaction.
3. I recognize stress from observation: is it drinking well? How does the foliage look? Are there signs I know about, with a known cause? Does it smell OK? In other words: if a tree suddenly changes behavior, it's worth investigating.
4. Yes. See below.
In the past 15 years I've learned to "read" foliage. Leafs tell us what is happening. It seems that there is a universal system in plants, because many of the signals overlap between species. I think this is because all plants work on the same principles. With trees however, we have to stick to the seasons as well. A pine with some yellow needles in autumn is logical; it's dropping the old needles. It helps to question if something is caused by weather conditions, or by human interaction, or if it happens randomly.
- In pines it's foliage color and type of foliage, if it's juvenile foliage and the tree is older than 3 years, than the tree is stressed. Yellow foliage happens with soil issues; often too wet. Brown = dead foliage, was too dry at some point. Purple = cold response OR soil too wet. No buds formed and random(!) needles in this year shoots going brown = water shortage during summer. Overall 'light green', possible micronutrient deficiency which can be tackled all at once by giving micronutrients. If certain branches go brown entirely, it's most often a root issue on one side of the container. Juvenile foliage on adult trees: too much pruning.
- In junipers it's foliage color and type of foliage too. Juvenile foliage happens when it's stressed, sometimes pruning, sometimes a lack of light, almost never root problems. If the tips die, water/soil issues. If entire parts go purple, it's sap flow issues or cold response, check the temperature and the wiring. If they turn from green to golden brown, cold response. If the foliage dries up and goes crispy while staying green, it's due to the roots being shut down either from damage (pruning, or some other thing that kills roots) or drought.
In general for conifers: lack of growth usually means root issues. If the tree has been transplanted this year, it can stall for a year if the root damage is substantial.
Special about larches: can drop all of their needles if it's been too hot and/or too dry. If this is the case, they skip a yellow phase and go brown/purple/green. Try again next year. Happened to me twice. Too much shade will cause something similar to happen on some branches, but the progression is slower (weeks/months instead of days).
In deciduous it's harder to tell, I don't own many deciduous trees but in general:
See tomato guidelines for foliage diagnostics.
For the people using biocides: if a tree is giving off mixed signals and you've been using a lot of biocides in the past two months, check the label of your biocide and see if you used it according to the label. Some antibiotics screw up the internal workings of a plant if they're dosed too heavily.
For the people with a good nose: a healthy soil should smell like a forest or rocky area after a rain shower, not long after watering. Or it should smell like the bag your soil came in. If it smells like farmland or 'muddy' the soil has been wet for too long. Take fertilisation into account of course.. If you just threw a bunch of poop in there, it's going to smell like poop.
I'm always open for corrections or discussion. I posted my own observations, feel free to take it from here.