In general, you cannot tell anything about the purity of a substance from its density alone.
However, there may be cases where contaminants are of uniformly higher or lower density than the pure substance. In those cases, a change in the density would indicate a mixture rather than a pure substance.
A measurement of density, among other measurements, might be something that one would look at to determine purity. A pure substance will have a certain set of properties. If the density of a sample is wrong, the sample is certainly not pure. If the density is right, you need to check other properties, too.
However, there may be cases where contaminants are of uniformly higher or lower density than the pure substance. In those cases, a change in the density would indicate a mixture rather than a pure substance.
A measurement of density, among other measurements, might be something that one would look at to determine purity. A pure substance will have a certain set of properties. If the density of a sample is wrong, the sample is certainly not pure. If the density is right, you need to check other properties, too.