Plant roots are usually in the bottom two-thirds of the pot, so do not water until the bottom two-thirds starts to dry out slightly.
For a 6-inch pot, stick your index finger about 2 inches into the soil (approximately to the second joint of your finger). If the soil feels damp, don’t water. Keep repeating the test until the soil is barely moist at the 2-inch depth. For smaller pots, 1 inch into the soil is the proper depth to measure.
When you test for watering, pay attention to the soil. If your finger can’t penetrate 2 inches deep, you either need a more porous soil mix, or the plant is becoming root-bound. The inbound roots blocks the water penetration deep into the soil.
It is recommended to water the plants at late evening and early morning to fend off evaporation at day time. So, avoid watering your plant at broader day light and fill up the brim of your pot and check until it slowly absorb completely, provided proper drainage holes.