Use the 'wet/dry' method. Feel the weight of a fully watered pot and then feel the weight of a dry pot. When the pot feels light again - water it again.
Don't stick your fingers into the soil, you'll damage roots, and if your hands aren't clean you can start introducing harmful pathogens and bacteria into your soil, and it's not a very good way of judging the moisture content of a pot. The dryness of the top couple of inches of a pot is no indication at all of the dryness of the whole pot.
This is the tried, trusted and proven method of judging watering schedules that pretty much all experienced growers use, except those newbies who stick their fingers down into the soil by 3 inches of course.