There are many visual cues that give you an idea of the overall health of your plants and shrubs. Knowing what to look for can give you an idea of the problem.
There are three major types of diseases; fungal, viral and bacterial.
As far as the fungal side of the diseases, they occur during hot, humid environments. They present themselves as brown/reddish spots on the leaves and flowers. Treatment includes fungicides and regular trimming and pruning of affected areas.
Bacterial diseases will also present themselves as brownish spots on leaves but they will emerge from the stems of the leaves or cuts on the leaves. Bacterial diseases have no cure but there are chemical preventative measures you can take.
Viral diseases also have spots on leaves but they will be more yellow in color. Viral diseases are transmitted chemically and therefore if a plant is infected, it is likely you will have to purge it. If you chose to trim and prune it, you will have to sterilize your shears every time you use them as they will carry the disease over to other parts of the plant and other plants as well.