You may water your plants with the best of intentions, but it's good to periodically check up on yourself, to make sure you're doing a good job.
One way would be to stick your finger into the ground right after watering. But feeder roots of plants are mostly in the top foot of soil, so that's the depth of soil that needs to be moist. And that's deeper than your finger can probe.
The next technological leap, one step up from using your finger, is using a trowel to check the soil. Now you can dig down a foot or so to see and feel for soil moisture.
But digging holes can become tedious, so it's fortunate that there are other ways to find out if your watering has been effective or if more is needed.
Electronic moisture meters are available through many local and mail-order garden supply stores. You merely push the metal probe of these devices into the ground, then read the result - dry, moist or wet - on the gauge atop the probe.
Be careful, though, because fertilizers can have some effect on readings - they can make the soil read drier than it actually is. Some fertilizers, especially chemical fertilizers, are like salts in the soil and salts have an affinity with water.
Remove and clean the probe after each reading.
A totally different approach to knowing when and how much to water is the indirect approach, by estimation. You can do this using a pan of water, from which water evaporates about as fast as it's lost from the soil through evaporation and plant uptake.
A 1-gallon paint can makes a good "pan evaporameter," but any large, straight-sided container will do. Fill it to within 2 inches of its top with water and place it in the sun. Measure the water depth with a ruler, repeating the measurement on a daily or weekly basis.
When your measurements show a loss of water, make up the difference by watering. If the depth drops a half-inch, for example, turn on your sprinkler until it brings the level back up in the can.
If you are using the measurement in your can to tell you when to dribble water on the ground from the end of your hose or from a watering can, count each 1 inch depth of water lost as equivalent to a half-gallon of water per square foot of area to be watered.
You could carry estimating one step further and just assume that your plants need 1 inch of water per week through the growing season. This is a very rough estimate, but if you religiously provide that much water, your plants will be pretty happy.