How to make artificial sea water
If you look at the (average) composition of seawater:

       

you will find that NaCl is the main solute of this complex solution. Its concentration is 10 times higher than that of the next constituents (magnesium and sulfate). So, it seems to me like a simple NaCl solution is a good approximation for your experiments.

Regarding testing salinity of water, I don't have a definite answer on that, I do have two suggestions to offer:


Measure salinity by electric conductance of the solution, e.g. by making a simple conductometer and testing it with solutions of known concentration of salt.
Measure salinity by the quantity of salt (solid residue) once all water is evaporated. You can speed the evaporation by heating up.