Is it obvious to an opponent that a Barbarian has entered Rage?

I had a look around but couldn’t find an answer to this. Is it obvious when a Barbarian begins to Rage? A canny opponent might know of a Barbarian’s rage ability and therefore, if they determine the Barbarian is indeed raging, employ strategies that make the Rage fade i.e. not attacking and using terrain, movement, etc to not let the Barbarian do any damage.

Is this possible to determine?