Usually, growers only know a plant is in ill health because it looks different. There are many visual signs that can be used to know if your plant has health problems or diseases. It's important to note that plants can show damage very quickly while recovery can take longer. This leads a lot of growers to overcorrect.
Most abnormalities in leaf color can be found in this article. This covers nutrient deficiencies and toxicities. If your plant shows other signs of damage it may be the work of Insects or other pests.
By Leaf appearance
Fan leaf color
The color and general appearance of fan leaves can provide a lot of information about issues involving nutrients. If the color is uneven it can be a sign of poor health.
Fan leaf patterns for diagnosing nutrient issues (click for higher res)
Leaves that are a shade of very dark green can be a sign of nitrogen toxicity.
"burned" leaf apex
The fan leaves will also show signs of any nutrient burn taking place. Nutrient burn symptoms occur at the tip of fan leaf "fingers" (the apex) where the leaf turns brown and crispy.
This can be a sign of both under and over-watering.
Claw fan leaves
The apex of the leaf clawing down is most often a sign of wind burn (airflow being too intense) or nitrogen toxicity
By plant appearance
If roots outgrow the pot they are in they become "bound". If this occurs the plant should be moved to a larger medium to continue growing. Constricting the roots will limit the overall size of the plant and may stunt it.
loss of stem rigidity
This can be a symptom of under-watering