There's a lot of reasons leaves can look droopy. Both over and under watering do that, but if you say it only does it when the soil gets dry, then maybe it is from under watering. For me, I know it's time to water if the leaves start to droop a little, but if I'm on the ball sometimes I can water just before that happens. Watch for the dirt pulling away from the edges of the container, and also after the soil dries out a couple times you should be able to know if it's time, from how light the plant and pot has gotten. If it's really light, or much lighter than normal, there may not be much, if any moisture in the soil. You can also stick your finger down into the soil to feel it, or invest in a $5 moisture meter, which usually work OK. The $100 ones are a lot better though.