For a slow leak however, use food coloring. Put 6 or 7 drops in the tank of the toilet (not the bowl). Use blue, the color of most toilet bowl cleaners. Wait a half hour to an hour.
Check the bathroom floor around the base of the toilet. If you see any blue, there’s some leaking. Check inside the bowl. If there’s a blue tint in there, then there’s leaking from the tank to the bowl.
A slow leak in your toilet can mean not just the waste of hundreds of gallons of water per year, but possible water damage and rot to floors, tiles, etc.