How do I know if the plant is hungry?
First signs of need are of the lower leaves, discoloration, distortion, or both. Pro's do it all by measurements, I'm an armature but I use organics so nute burn is a low risk for me. If you're on chemicals you need to study how to properly add them, also if I where you Id test the soil and see what it needs.
I assume you can purchase a test kit at your local hardware store, one that includes pH N Pa K test for all the basic nutes.
For example I look at the plant and ask myself should that be like this? The right question to ask about your plants currently is, "Why are the leaves curling up?"
Now if you have the correct question its time to start your search for the correct answer, ya know dude?