to check how much electrolyte is present in the battery
a hydrometer checks the state of battery charge using specific gravity.
Hydrometer.
By using refractometer or Hydrometer
With a hydrometer which you can buy at any auto parts store.
a Hydrometer is used to measure the density of a solution, by the use of a scaled float. typically it is used for battery acid.
fully operational
Hydrometer
Yes
That's a 'hydrometer'.
It is an instrument to measure or gauge density of gravity.
Watch this: https://www.youtube.com/watch?v=SRcOqfL6GqQ
For checking specific gravities of liquids such as battery acid and antifreeze.