Let us count the ways – (some) Answers
– see how well it floats objects (Moderator-80)
– measure the specific gravity with a urine dipstick (Moderator-80)
– put a goldfish in both, see which survives (Moderator-80) [along those lines, water a plant with it, or see which one a pet will drink]
– see which conducts electricity better (Moderator-80)
– heat it up or let it evaporate and see if there is a residue of crystallized salt (Moderator-80)
– put a cathode and anode in it and see if chlorine gas is released (Moderator-80) [interesting]
– put salt in both and see which one precipitates out first (Moderator-80) [yep, the one that has already dissolved the max amount wont dissolve any more]
– put a little more liquid in and gauge the meniscus (Moderator-80)
– cool both, the salt will come out of solution, that might be the same as a prior answer I gave. (Moderator-80)
– measure the osmolarity (Moderator-80) [say what??? ]
– have someone else taste it and report their findings (Moderator-80)
– Cut your finger and pour some water from each glass on the wound. (ronrsr)
– Put some in your eyes. (ronrsr) [ I never swam in the ocean. Does it sting?]
– take 2 portions of unsalted meat
pour one glass on each piece
rinse it off
take both pieces to the Godol Ha Dor and ask which is kosher (Moderator-80)
Also, saltwater freezes at a lower temperature and boils at a higher temperature than fresh water.