How to determine externally that a whale is a mammal? Mammals have external qualifying characteristics like body hair, ear pinna, etc. But a whale has none of that. So, can it be identified as a mammal just by external observations?

A whale has a horizontal tail fin which is a typical feature of mammalian marine animals see link. Mammalian sea animals typically have a vertical movement of the tail fin for swimming. Fishes like sharks have a vertically oriented tail fin and move it side-ways for locomotion.