The usual advice is to test the top of the soil. Not bad, but accuracy depends on many factors. When I worked in a nursery I learned a different method, lifting the pot and judging dryness by its weight.
Of course, knowing the heft of a soaking wet pot is the first step. Easy to do. Then pick it up occasionally, just to get the sense of the process. If your contain is too large to pick up, tip it to one side or the another. That should give you a weight estimate as well.
When to water? Depends on the plant. Some, like roses and fuchsias, can handle frequent drenching as long as the potting mix is loose. Others, like lavender, rosemary and succulents, need to dry out a bit.
As always, know your plant and its likes and dislikes. Most like a middle ground, slightly dry before watering. Always a safe bet.
