Nutrients are leached out by watering. Roots can be examined by loosening the plant from the container. Water the soil before examination. Give a gentle tap on the rim of the pot to loosen the plant. If there are mostly roots and little soil, the plant is pot-bound and it is time to repot.
Roots growing out of the bottom of the pot is another sign that the plant is pot-bound.
Repotting should be done in spring, when the plant begins active growth. If the repotting of a flowering variety is being considered, wait until after it has flowered. The next pot a plant goes into should be no more than two inches larger in diameter than the current outgrown pot. A small plant should never be potted in a large pot. If this is done, water is retained in the soil because roots are not absorbing the water.