Leaks can occur in many different hidden places around the home, making them hard to find. If you find a leak, there are many do-it-yourself videos online that can show you how to fix most problems. If you don’t feel confident in fixing the problem yourself, there are many plumbers that specialize in both leak detection and remediation.
Before you do anything, you may want to first check your toilets for leaks. It is easy to do, and they are often the culprit. To test for toilet leaks, add a few drops of food coloring to your toilet tank and wait 10 minutes. If the color shows up in your bowl, then you have a leak allowing water to flow from the tank to your drain without ever flushing the bowl.
If it's not your toilet, here is a great resource for finding a leak in your home.
If you can't find the leak yourself, you may have to call a plumber that specializes in leak detection and remediation. They usually have some special devices such as moisture readers and infrared cameras.