see how well it floats objects
measure the specific gravity with a urine dipstick
put a goldfish in both, see which survives
see whick conducts electricity better
weigh both on an extremely accurate scale (im not sure about this one)
heat it up or let it evaporate and see if there is a residue of crystallized salt
put a cathode and anode in it and see if chlorine gas is released
put salt in both and see which one precipitates out first
thats enough im busy