Do scientists use the measure of density to tell if something will float?

yes. for example; Ice is just frozen water, right? So why does ice float in water? When we find the mass of an ice cube and divide that by its volume, we come up with about .92. Ninety-two hundredths is less than one, so the density of ice is less then that of water. Ice floats because the ratio of its mass to volume is less than one.