This is really an area that needs to be done by an expert but there are many signs that indicate a problem or potential problem. It is important to understand that different types of trees have different characteristics and problems unique to the variety. That said, I will give some general advice that are “red flags” to a potential problem.

Cracks in the limbs:
This sounds pretty basic but I am amazed at how few people really look closely at their trees.  It is often obvious when a limb is splitting open but not always. Small cracks in main crotches can sometimes ooze water or pitch (sap).This can happen suddenly or take some time to develop. If you see cracks or openings, call an Arborist immediately.  That may be the only warning you get.

Cracking noise:
I have heard and been told by people that they heard a loud crack, but could not see anything. Sometimes the crack is on top of an upper canopy crotch or running along a large horizontal limb. Don’t ignore this obvious clue.

Change in the angle of the tree trunk:
Many times in my career, clients have called me to say “it seems like the tree is leaning more than it was before”. Sometimes it is a persons fear or imagination but many times there was a weakness underground that caused a change. The large Stone Pine in Blaney Plaza was saved because a city worker noticed a change in angle.  Sure enough, when we looked close, the whole root ball was coming up and this huge tree was on the verge of going over. Sometimes cracks in the soil are a good indicator of movement. Also look for a separation in the soil line around the base of the trunk. In rainy weather, the soil becomes muddy and these clues are often harder to read.

Drooping limbs:
Do some branches seem lower than they did a few weeks ago? Is there a dramatic difference in the way a tree looks?  There is a situation that is very common during hot weather called “summer limb drop”. A drying out of the limbs can cause a sudden limb failure, even on a windless day! Some of the species that this happens to include; Monterey Pines, Italian Stone Pines, Valley Oaks, Eucalyptus species, and large Ash. This can happen on other varieties of trees as well but these are the most common in the Bay Area. (Correct me if you think of others and I will list it as well).

Mushrooms at the base of the tree:
Some types of wood decaying organisms will push up fruiting bodies or mushrooms.  It is not the actual mushroom that is the problem but what causes the mushroom. Rotting roots or trunks cannot always be identified at a glance. There are techniques that help us to determine the extent of the decay and likelihood of total tree failure and this inspection needs to be left to an expert. However, if you see these clues, get help before it is too late.

Fungal bodies on the trunk:
Just like the ground mushrooms, there are many types of growths that indicate decay.If you see anything unusual on the trunk of the tree, other than moss or lichen, then it is a good idea to get it inspected.  It may not be a big problem or it might just be the flag to a bigger problem. Black balls on the trunk and limbs that look like charcoal are a big indicator to a possible disaster. Get the tree checked out!

Girdled roots:
This is a tough one to identify as a problem but sometimes there are surface signs to girdled roots. When roots   twist around each other they can sometimes cut off normal root development. If a tree was in the pot too long, then the roots can grow in a circle, rather than spreading out. Sometimes you can recognize a girdled tree by a lack of a root flair. If the trunk seems to indent at the ground, that may be a clue to girdled roots. Canary Island Pines are one type of a very fast growing tree that we see failing from having lived in a pot too long. The fast growth on top can exceed the root development and a girdled root ball can cause the tree to snap off at the ground.

There are many other signs of potential problems such as hollow trunks, cavities, past trunk damage, insect activity, improper past tree work, etc., but these are really areas that you would be guessing at. The main point that I am trying to make is that if you notice anything that worries you then you should get the tree checked out.