There are two ways to find out that your toilet is leaking. The first, and most obvious, is water pooling around your toilet. The second, less obvious, is that your toilet tank refills on occasion even when not flushed - it's doing this to replace the water that has leaked.

Once you know that your toilet is leaking, you need to find the source of the leak. To do this, put a couple drops of food coloring into the tank water and let the toilet sit without use. If the water in the bowl turns the color of the dye, the flapper valve inside the tank (it's the thing at the very bottom of the tank) is leaking and needs to be replace. If colored water drips down the outside the toilet, you need to replace the seal between the tank and bowl. If you find colored water collecting on the floor (and no signs of leaking on the outside of the toilet), this indicates a faulty wax seal beneath the toilet; you'll need to pull up the entire toilet and replace the seal. Depending on how handy you are, this might be the time to make a call to a plumber.