Overwatering is an issue of there being no oxygen available to the roots, that's pretty much it. Underwatering and overwatering will both result in droopy leaves. Water your plant, then pick the pot up and get used to this weight, once it's drunk most of the water you will be able to notice a big difference in pot weight, it's vague at first but you'll soon work out what you're doing, no matter when you last watered it, if your pot is that dry weight, then water it again.