How to determine if the soil has enough nutrients and also how to know if I am under watering or over watering the plant? Also, why some tomatoes are rotting at the bottom of the plant? How can I get a test kit for soil?
A soil test is needed to determine what nutrients your soil has or what is needed to amend it.
Your County Extension Office can help you with a soil test for a nominal fee.
Here is a link to help you locate your nearest office.
These links will help you with your Tomato plants.