How do I know if water in supermarket's seafood tank is salty enough?
There are several ways to measure water salinity (water salt content), most are impractical for that situation. A very effective and quick way to measure salinity to use a salinity meter, these are small electronic devices which measure the conductivity of the water. You just dip it in the water and take a reading. More salt = more conductivity. 

These can be bought at many pet stores that sell salt water aquarium equipment and fish. Good ones aren't super cheap, expect to pay about 100USD. There are super cheap ones on the market but they generally don't work very well. 

Keep in mind salinity is only one factor, there's the purity of the water, oxygen content, temperature and other factors you need to consider as well. Before they are put into the tanks how were the shellfish collected and treated during transport? 

The fact is even if you check the salinity and it is good (I'd be surprised if it wasn't) the other factors make it impossible for you to know how good the shellfish is. This is why tax money goes into food safety inspectors - so you can shop with confidence. Look at the food safety rating of the store or chain, see if there's any reputation based ratings from customers and base your choice on that.