Modern flush valves have a built-in overflow that sends the water into the bowl so they don’t require the old-fashioned overflow pipe out of the building. If you are not seeing any water going into the bowl there can’t be much water dripping. However, it is hard to see the water going into the bowl as it doesn’t drip but trickles down the back of the bowl and you often only notice because the area where it is trickling gets discoloured over time. As Eddie says, these things have a limited life and do need replacing, it is a question of how much water you are losing and how much it annoys you as to when you replace it.