##Legends continuity

The Star Wars Wikia has [a page on the Death Star][1] that states the laser of the Death Star was tested a couple of times.  Once on an Imperial Star Destroyer and once on Despayre, the planet where the station was constructed.

An extract from this page:

>In 0 BBY, Princess Leia Organa of Alderaan, perhaps while the Alliance Intelligence investigated the existence of a superweapon, was informed about it by a soldier she rescued on Ralltiir. The existence was justified by Intelligence on AX-235. Rianna Saren's report on the Destruction of Despayre confirmed the power of the battlestation's superlaser.

>In addition, Rahm Kota issued a reconnaissance mission to the station to gather intel on its primary systems, in particular its superlaser weapon. X2 personally volunteered for the mission, feeling indebted to trying to stop the Empire he unwittingly aided in creating. At this time, the tributary beam had just been shipped to the Death Star, which X2 discovered via a transmission from X1 intended for the commander of the Death Star. X2, under Kota's command, then made an impromptu superlaser test on an Imperial-class Star Destroyer that the Rebels had hijacked and scuttled earlier, causing them to realize the unimaginable destructive power behind the weapon.


  [1]: http://starwars.wikia.com/wiki/DS-1_Orbital_Battle_Station