A plant can usually recover from not being watered enough, however if you over water the plant, it is a goner. You can go out and purchase an expensive moisture reader, but the best indicator of moisture level in the soil is your index finger. If you can stick your index finger down into the soil and feel moisture, then the plant will be OK. If the soil is dry, it is time to add water. Not sure? Then it is better to let the plant tell you it is thirsty; the leaves and stem will droop. Give it a drink of water and viola! Happy plant.
Check out the pictures of these tomato plants. In the first picture I let them get too dry. I watered them and then they pepped up. These tomato plants are currently planted and thriving in the soil. Getting a little dry is OK, but be careful not to water too much.