How to use a refractometer
A seawater refractometer uses the refraction of light in liquid to measure specific gravity. Place a few drops of water under the refractometer lid, close the lid, and point it at a light source. Look through the eyepiece for a change in color created by refracted light. The line will run through a chart inside the eyepiece that indicates the specific gravity.
A digital refractometer is even more precise than a mechanical refractometer.
Another option is a digital water tester, a handheld meter that combines a thermometer and salinity tester to make water changes a breeze