At Marineford, specifically in Episode 478, Luffy demonstrates he has no control over it. It fires off randomly when his emotions are at their peak.

This scene is the catalyst for the training arc. Luffy realizes that despite his Gear Second and Third, he is too weak to save his friends (as seen later with Kuma at Sabaody and Ace at Marineford).

Immediately after Luffy uses the Haki, the reaction from the veteran pirates confirms its nature:

This validates Luffy not just as a rookie with a Devil Fruit, but as a chosen one with the "Qualities of a King."