Other than root problems, your indoor plants will seldom be troubled by diseases because the lack of humidity in the home prevents them from developing.
Don't be afraid to remove the plant from the pot and examine the roots to see if they are too dry, too wet or diseased. Invert the plant and lightly tap the edge of the container on a table while holding the plant and soil ball. You may find the soil mass is completely taken over by roots and the plant needs repotting.
If you suspect root rot problems, remove the rootball and check the roots. Healthy roots and root tips are white or cream-colored. Rotted roots are black and appear slimy.