"My plant has yellow leaves. Am I overwatering it or underwatering it?"
I've heard this question a lot in my career as a horticulturist -- and interestingly, it's not nearly as easy to answer as it sounds. It seems like it should be pretty straightforward, right? But what makes it tricky is that overwatering and underwatering a houseplant starts out the same. When a plant is too dry, its roots start to die because there's not enough water to support them. And when a plant is too wet, its roots start to die because there's too much water and not enough air in the potting mix.
So how do you know? The best way is to feel for soil moisture. The average houseplant likes to be watered when the top inch or two of the potting mix is dry to the touch. And if you have a saucer or dish underneath to catch excess moisture, most plants don't want to sit in water for more than 45 minutes or so.
Happily, you don't have to worry too much. If you forget to water every now and again and your plants get a little dried out, it doesn't cause long-term damage. The same goes if you overwater from time to time. The time to worry is if it becomes a consistent pattern; that's when you start to see more leaves turn yellow and the plant suffer.