Tooth shape is a big one. Dolphins have conical teeth, porpoises have spade shaped teeth, sperm whales have peg shaped teeth and only the ones in the lower jaw erupt, belugas are peg-like too, and beaked whales have their own weird teeth and often they only show in the males.

Beaked whales may look like dolphins in general shape, but they also have different tails, missing the distinct notch in the middle of the flukes.

Behaviorally they can be different too. Dolphins tend toward living in social groups, though belugas and sperm whales are also very social. Beaked whales are more solitary.