There are a number of methods to determine when to water an indoor potted plant. The method may also vary with the particular plant species and the way it is potted and the potting medium used. But there is no particular right or wrong way. Some folks have more success with one way rather than another.
That said, the most common method is to allow the soil to get dry to a certain depth before watering. Some plant species will need water as soon as the surface feels dry while others, primarily succulents, will need the soil to dry much deeper onto the pot before watering. There is no single rule for just how deep the soil should get dry. The concept is that the soil will dry from the top down so checking the top of the soil will give you a better idea of when it needs water.