In fact, Couch's kingbird was considered conspecific with the tropical kingbird until 1979. The easiest way to distinguish between the two species is to listen for their call: Couch's kingbird has a raspier, more complex call that is more varied in pitch.
Couch's kingbird was previously thought to be identical to the tropical kingbird until 1979. To distinguish them, one should pay attention to their calls: Couch's kingbird has a raspier and more varied call than the tropical kingbird.