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.

You cannot (technically, yes, you could, but practically, no)

First, a store will not let you tamper with the tanks; either by taking sample or by prodding it with some tools (unless you are a real food inspector)

Second, who are you to know if the water is "good enough" ? Are you a food inspector? what guide will you use.

Third, Stores are not set up to sell you bad products; it is a question of trust; if you don't trust the freshness of the produce, don't buy it; find another store.

As with your other question (linked in this question)

what are you looking for ? what is your goal ? Even if storage is good, it will not prevent bad product (for example, 1, 2 bad clams in a bunch).

Do you have insight that the store is not following proper storage method ? (according to local regulations)

Its pretty simple: if the shellfish are all dead and the tank is smelling like a Marseille sewer after the bouillabaisse festival? Then the water is too fresh.

Many shellfish will tolerate a decent range of salinity. They have to as in the wild salinities can vary wildly with the tide and discharge from local rivers.

For freshness/animal welfare (they are the same in this case) it is far more important how well the water is filtered and areated. Oxygen is hard to measure but if the water is flowing and being pumped around should be broadly fine. Filtration is something you can test with a aquarium-test for nitrogen. You'll need a sample of water though and the supermarket will be hesitant to give you this.