Density and relative density of a liquid can be easily determined by using relative density bottle. A bottle has a specific volume and a stopper which has a fine central hole to ensure that any excess liquid is risen out of the bottle after filled it with a liquid to be measured.
Density of a liquid can be measured by taking the ratio of the mass of liquid to it's volume.
Relative density of a liquid can be calculated by taking the ratio of density of liquid to that of water. For more details see video.