I think you might find a hydrometer useful.
These simple, floating devices are used to measure the densities of liquids.
Probably most familiar in home brewing for measuring the sugar content ot the wort / must to be fermented or the alcohol content of the finished product !
They are small, floating glass devices - a bit like an anglers flaot. the more dense the liquid, (because of dissolved sugar or salt) the higher the float will sit. The less dense the liquid (eg because of more alcohol, which is less dense than water), the lower it will sink.
You will need one calibrated for salt (sodium chloride or NaCl) and these can be sold by aquarist supplies - they are used to measure the salinity of marine aquaria.
There's some
here for less than a fiver.
You may need to check the calibration
(I don't think you need one calibrated in Moles per litre !! (Mol/l) )
Failing that - surely the "recipe" should give you the concentration of brine required by saying "X grammes of salt per litre" or "Y ounces per gallon"?