You can detect leaks in your toilet by removing the tank cover and adding some food coloring in the tank just before you go to bed. If you have even a small leak, the bowl will have the food coloring in it by the next morning.
The most common leaks occur under the flush valve or flapper valve. To determine if this is your problem, take the tank cover off and listen closely for running water. Call a plumber or you can try to fix it yourself following these steps:
Reach under the tank and shut off the water.
Flush the toilet.
No water should run into the tank if the valve is shut off tightly.
Remove the rubber flush valve and clean it with warm soapy water.
Try putting some Vaseline on the bottom to improve the seal.
Turn the water back on and test for leaks.
A new flapper valve can be purchased at a local hardware store if necessary.