A hydrometer will float in a sample of your tank water, and depending on how high it floats in the water will give you a reading of how buoyant your water is. When purchasing a hydrometer, make sure it works in the 1.2 to 1.3 or more range, otherwise it will not be giving you usable data at all. Aim to keep specific gravity levels between 1.265 to 1.285 at all times.
Length = 190mm
Divided in 0.002 SG