The density of a substance is the mass of each unit volume of the substance. The density of liquids does not vary much with changes in pressure and temperature. The density of gases on the other hand is easily influenced by changes in temperature and pressure. The density of any gas can be increased or decreased to a great extent by changing the environmental conditions.
All liquids and gases do not have a unique density, one that is not shared with other liquids and gases resp. This makes it very difficult to identify them based solely on their densities. Also, to identify fluids based only on the density would require a very large database with densities of all liquids and gases under all environmental conditions with varying temperatures and pressures.
This is nearly impossible to do. This makes the use of density only as means to give a rough estimate of what a particular substance could be and then other tests are performed to uniquely identify it.