To check for water loss, remove the toilet tank cover and look at the level of the water in the tank. If the water level is at the top of the fill tube, the water is overflowing into the tube and causing the toilet to create a constant demand for water. You can repair this condition be adjusting the float. The water level should never be closer to the top of the fill tube than one inch.
If the level of the water in the toilet tank is correct (not closer than one inch to the top of the fill tube), the other major leak source is the bottom flapper. If the toilet cycles or you can hear the toilet trying to fill up with water, then the bottom flapper is not properly seating. The best solution is to buy a new rubber seat seal to insure proper seating on the bottom drain hole.