With a well-draining soil it is very hard, not saying almost impossible to overwater. By well-draining, I mean that water should be able to drain freely out of the pot almost instantly after you water it. It should not be able to form puddles on the surface, not even for a second, and not even when you pour a whole can over it at once. And yet, the mixture should be able to hold just enough water make it through the day on a hot and sunny summer day. The soil should also be able too keep these characteristics for a number of years. If this is the case with you, and only then, we are talking about a wel-draining mixture.