Using estimates and apporximations we can work out (within a reasonable accuracy) how much sand there is on the earth. While this is not exact we can use it to roughly compare with stars. Using ultra-sound models of the universe and advanced telescopes to detect them the normal way we can also work out aproximatley how many galaxies there are. I dont know about all the sand on the earth, but we can see that there are far more galaxies out there than grains of sand on our beaches, and in each galaxy there are billions and billions of stars, so i would definitaly agree that there are more stars than grains of sand on the earth.