Superficially, Brachiopods look a lot like bivalve Mollusks (clams, etc.). There are several ways to tell the live creatures apart. The Brachiopod will usually have a pedicle, a long stalk attached to one of the valves, and not much meat inside, just a lophophore. A bivalve Mollusk has no pedicle and lots of good, juicy meat.

None of these do much good when all you have is a shell, and the shell has been a rock for hundreds of millions of years. So you have to look at the symmetry of the shells.

A Brachiopod is equilateral but not equivalved. That is, the two valves (separate parts of a 2-part shell) are not the same, but each valve is symmetrical from one side to the other.

A bivalve Mollusk, like a clam, is equivalved, but not equilateral. The two valves of a clam shell are mirror images of each other, but each valve is asymmetrical to itself.

The exception to all this are the Ostrea, or oysters, which have no symmetry whatsover.