Grab a handful of soil and squeeze it with your palm. If you can make it into a ball that doesn't fall apart in your hand you are watering enough.

Hi Colleen, I think you are searching for a moisture meter...

Here is a link that might be useful: moisture meter

The best moisture meter in the world is the weight of the pot.

Pick up the pot and if it is heavy it has water. If it is light weight then it is dry and needs water.

Forget moisture meters they do not work. waste of money. Only the weight method works.

I'm with koreyk. Once you get used to the weight and look of your container plants, you will be glad you didn't waste your money on a gadget.

Observe your plants. You will learn to tell when they are beginning to feel the strain and need watering. Don't water by rote. Do it when your plants tell you they need it.

I water by humidity, and temperature. Depending on whether or not they are in full sun or part sun. What soil did you plant with? How do the plants look?

Also, what plants are you talking about. Some plants require very little water until bloom while others require more frequent watering. For instance, as a rule of thumb I make sure I water my tomatoes with a minimum of 2 inches per week while fruit production is going on.

All good advice you have gotten so far. I would add that at the beginning your plants may need watering twice a week and by the middle of the season when there are many more roots it could be twice a day. You just have to get used to what they need depending on all the factors.

One gadget that is very useful though ( and cheap ) is a rain gauge. The local weather caster could be many miles away when they tell you you got half an inch of rain when it barely sprinkled at your home. I have one that looks like a frog.

Look to see when the soil is pulling away from the sides of the plant. If yes, then you need to water.