How do you know when a plant is hungry: Many times the color of the foliage will be the first clue. The leaves may not be glossy, the color will fade and the plant won't grow as fast, if at all.
Sometimes it is just a certain element that is missing. Most everyone can recognize the lack of iron. The leaves turn yellow, although they're still alive, and the veins are green. Other mineral shortages are possible and you should always bring several live leaves of a sickly plant to a California certified nurseryman at your local garden center for a diagnosis and cure.
Before feeding any plant, water it so that it is not thirsty. If you have any qualms about how much fertilizer to use, use less and be safe. You can always feed at a lesser rate and then just do it more often if you need to. If you have been feeding your plants well and you're not getting the results you want, then certainly consider that the soil may not have enough humus in it. The bacteria in the humus helps turn the fertilizer into a form the plant can use.
Though the majority of plants like a balanced fertilizer and generally you can use a basic food for almost everything, some plants do better if they have a specialized food. Other plants, such as the protea, do not like any fertilizer with phosphorus, even though this is a requirement for all other flowering plants.