Jim,
Put some vegetable dye in the toilet's water tank. Come back in an hour and look in the toilet bowl. If the water is colored then you have a leaking flapper valve. Check the valve seat for dirt or burrs and replace the flapper. The flapper is the usual fix for your problem If the water in the bowl is clear then look for moisture under the water tank at the point where the ballcock assembly passes through the tank and where the tank bolts to the seat. Some of these gaskets may be leaking. If you don't see moisture on the bottom of the tank you could have a leak where the seat meets the floor. Go to the basement or crawl space and look for moisture around the soil pipe, Check to see if the float is out of adjustment
Good luck, Dave M.