Can I assume fish is wholesome if I purchase it frozen?

Not necessarily. Fresh catches are often processed and frozen right on the fishing vessel, though fish can spoil if it thaws during transport or is held at warm temperatures for too long.
 
If the packaging is transparent, look for signs of frost or ice crystals, which denote the fish has been  stored a long time or has thawed and refrozen. If you see ice or crystals, select another package. Don't buy frozen seafood if its package is open, torn, or crushed on the edges, and avoid packages positioned above the "frost line," or top of the freezer case. In general, freezing destroys parasites but not bacteria and other micro-organisms. The only way to ensure fish is safe is to cook it thoroughly.
 
Sources:
"FDA "How Can You Select a Safe Seafood?"