The most common way to kill a houseplant is to water it on a schedule. Plants do not drink on Tuesdays. They drink when the soil dries out, and how fast that happens depends on the light, the temperature, the pot and the season.
The reliable test costs nothing. Push a finger two inches into the soil. If it comes out dry, water. If soil sticks to your finger, wait and check again in three days. In winter that gap can stretch to two or three weeks for the same plant.
When you do water, water properly. A splash on the surface wets the top inch and leaves the roots dry. Pour until water runs out of the drainage hole, then tip out whatever collects in the saucer. Roots sitting in standing water are the fastest route to root rot.
Signs of too much water look a lot like signs of too little: drooping leaves, yellowing at the base, a general sulk. The difference is in the soil. Wet soil plus drooping means the roots are struggling to breathe. Dry soil plus drooping means the plant is thirsty.
If your pot has no drainage hole — as some of the indoor Willow planters deliberately do not — keep the plant in its nursery pot and lift it out to water. That way you control exactly how much sits at the bottom.