After planting, you will want to keep moisture in the soil on the wet side until the plants start to emerge. Once they have, it's best to wait for the soil to dry slightly before you water again. One way to check water is by looking at the surface of the soil and seeing if it is wet. A wet surface will be darker than a dry surface (shown in the picture). You will also get a feel for how much water is in the soil by lifting on the pot a bit to feel its weight. Heavier pots mean that there is more water in the soil. How often you should water really depends on how fast the plant uses the water in the pot. A big plant will use the water faster in comparison to a small seedling.
In Pots Vs. In Ground
A good start is thinking about what the plants normally thrive in. In general the plants that are planted in the ground will have the advantage of water availability from the ground and a slightly dry soil on top. When you dig down into the ground and the first inch or so is on the dryer side but as you dig further down you start to notice the moisture. This is optimal for plant growth and disease suppression. Apply the same concept when deciding if you should water your plants. When growing in the ground or in pots you also have to think about how much of a root system your plant has. If you plant a seed and it sprouts, usually the roots on that little plant are just about as big as that shoot that you can see. If you only see an inch or so, then it's fair to assume that the roots only go down about that far. You just want to make sure that the soil around the root system does not experience a point where it is bone dry.
The Real Issue
My experiences have taught me that many times growers will find a crop that is looking like it's wilting near death with water logged soil, they will conclude that the crop was overwatered. In some cases that may be true, but more often prior to the overwatering the soil had dried out completely which caused damage to the roots. After finding the crop so dry, the grower waters the crop heavily and that's when the fungal problems arise. You can think of the plant as having a water uptake problem vs. a water availability problem. One thing I really like about CowPots is that unlike peat pots which wick water out of the soil and into the pot, CowPots have a bit of water holding ability but does not wick water out.
Overwatering and Disease
To understand the concept of overwatering, we normally think of plants as releasing oxygen and consuming carbon dioxide during photosynthesis. This is true and furthermore the roots actually need oxygen in the soil to thrive. If we were to constantly water a plant then we are reducing the amount of air in the soil and suffocating the roots. Constant moisture also creates a perfect environment for bad fungi and bacteria to grow that can harm roots.
Experience Takes Time
Knowing the pants will help to make watering decisions. Many herbs for example like to grow in soil that is very well drained. When growing them in containers I like to use a soil like Vermont Compost Fort Light. This soil has perlite in it which are those little white pebbles in the mix, they are actually a volcanic rock that is expanded by heat. The addition of perlite is a great way to make a soil drain better and add air space. I use something heavier when growing something like perennials, annuals and vegetable plant that I think will use a lot of water. In general, you can look at a plant and judge how big it is in comparison to the size pot it is in, this will give you an idea of how fast it will use the available water in the pot. Keeping track of what plants you grow and how often you water them can help keep track of what worked and what you can improve on.
The last point I'll touch on is that you should focus on selecting varieties that suite your watering habits and climate. This goes for your garden and your landscape. I like that Hart Seeds has a large selection of varieties. If you live in an area with a lot of rain, then plants that don't do well in wet soil may not be a good fit. This takes time to learn and feel free to send any questions you might have to me Jon@TheOnlineGreenhouse.com.