Dude I learned this to a tee on my first grow this summer. This is tough to get down as each leaf has say 5 fingers. Each finger has several ways it curls depending on whats happening. I am a novice but I noticed as i watered too much the leaves first curl at the tips of each finger. Then each finger will curl long ways. It remains green and the leaf skin looks flush green and full. If this occurs for several weeks then they drop sets of leaves throughout the plant. Yellowing on each of them and it looks terrible. If its too hot in a room a whole fan leaf will curl upwards. I wish I had a few examples, as I had trouble with watering.
You MUST (like another user said) not get into a habit of watering every few days. You MUST water only when they need it. It is better to under water and then fix the problem then it is to over water and have them drop sets of leaves.
I did this to a whole crop and went nuts, thought it was fetilizer issues then burned em and then realized it was over watering then got it under control. Remember that it is ok to let the soil get dry for a day. The plants will speak up and ask for water. if you over watering they usually let you know when its too late.
The only plant I Failed to over water was two OG TANGERINE. For some reason the more over watering I did to them. They remained calm.