One of the most important things is the reaction test:
Place your hand between the clam and the light. If the clam reacts to the sudden shade by closing up, it’s at least reasonably healthy. If the clam responds very slowly or not at all, leave it. Clams have a type of “spring” that forces them open; closing is an active mechanism and is the sign of a healthy clam.
Also, be on the lookout for any bleached or torn sections of the mantle, as well as making sure the mantle extends beyond the shell. However, note that the mantle of Hippopus species does not extend much past the shell. It’s also worth noting that T. gigas naturally has uniform clearer spots in the mantle; these are totally normal.
Look at the mouth (incurrent siphon). If it is very wide and gaping, I would not recommend purchasing this clam.
Inspect the clam for parasites prior to adding it to your reef. Snails can hide under the mantle as well as other hitch hikers that you might not want. Also check the shells for holes this is also a sign of parasites.
Make sure the mantle is rich and deep in its color with out any white spots that break the pattern. The flushing of zooanthellae is a definite sign of a clam on its way out.
There are still occasions where even though all of the positive signs mentioned above are shown, other possible red flags may go unnoticed. Sometimes the tank inhabitants can also offer premonitions of a clam that might otherwise appear completely healthy.
Look for a shrimp clinging patiently on the shell. Hermits or bristleworms congregated at the base of the shell near the byssal opening. These are telling signs of impending internal deterioration that aren’t indicative by the clams appearance.
Finally, consider size when purchasing a clam. Very large clams can sometimes have trouble acclimating; on the other hand, very small clams are obligate filter-feeders until they get larger. The safest bet is a good mid-sized clam. You also want to consider adult size: T. gigas, T. deresa, and T. squamosa all get very large and grow quickly. It would be irresponsible to house them in an environment which they will quickly outgrow.