@spec behaviour_implemented?(
  implementation_module :: module(),
  behaviour_module :: module()
) ::
  :ok
  | {:error, :not_a_module, value :: term()}
  | {:error, :behaviour_not_implemented}

Determines whether a given implementation_module is both a module and implements the behaviour required by behaviour_module.

Unfortunately, Elixir doesn't have an is_module guard, and you have to use is_atom, as Modules are considered atoms.

iex> is_atom(SomeModule)
true

Elixir will show you at compile-time (via a warning) if you haven't implemented a required function in a Behaviour. However, there is no way to determine whether or not a given Module implements another Module's Behaviour.

Return values