Moisture meters are available to determine the amount of water in the soil, but they are not reliable. The amount of fertilizer in the soil will influence the reading. If you fertilize heavily, the meters tend to read that the soil is moist even when the soil is dry. For small plants you can tell when to water by picking the pot up. Water weighs more then air, so a dry potted plant will be lighter than one that has recently been watered.
Tap water is acceptable for watering most plants.
Some plants are susceptible to fluoride injury from treated water. Many susceptible plants have long slender leaves such as dracaena and spider plant. Injury is characterized by brown spots along the margin or leaf tip. Fluoridated water should be allowed to sit at room temperature over night before using. Potting soils containing perlite can also cause fluoride injury.