You must know what the plant looks like when its healthy, flowering time, color, structure, and leaf size.
Should know plant stresses such as climate, lack of moisture. Look at the leaves as a barometer. The leaves tell a lot about the plants condition. Its good to have a magnifying glass.
Branches and stems should be looked at for swelling, bumps etc. Roots should be looked at for insect damage. Also look at other plants in the environment: growing conditions, maintenance, has it been pruned lately, animal damage, sunlight etc.
Also look into:
* vehicle traffic, construction, gas leaks, excavation, spraying in the area, spills of some kind.
Leaves
If the leaves are eaten or chewed: beetles , caterpillars, bees, earwigs, or slugs can be responsible.
If leaves are mined: larvae (sawfly), moth larvae
If leaves are distorted, puckered or looking too small or too many: fungus or bacteria blocked the system, aphids, spider mites, herbicides
Spots on leaves: bacteria, fungal infection, insect sucking on the underside of leaf, if spots are orange in color it could be rust.
If leaves are wilting: lack of moisture, too much moisture, nematodes attacking roots, shade plant stuck in sun, lack of sunlight, galls, fungus or bacteria blocking up plant
Grayish powdery mold on leaves: powdery mold
Evergreens
Evergreen brown and withered: salt toxicity, dried out
If leaves fall the tree when they aren’t supposed to: drought, fungal diseases, black spot, root problems, over fertilization, stress (transplanting or drought stress), caterpillars, spraying.
Honeydew dripping from tree: aphids, scale
Sooty mold on trees: caused from honeydew excreted from insects.
Trunks, Stems and Branches
Small holes in stems: woodpeckers, beetles ( bark beetles), borers.
Sap coming from stems, tiny bumps on stems: scale.
Woolly bumps woolly scale, woolly aphids.
Swelling on growths: insects, galls, bacteria, funguses, infections, dried out plant that has cracked or dried.
Tents or mats on tree: webworm, tent caterpillars, forest tent caterpillars.