Was the Death Star ever tested?

We know the first main use of the Death Star was the destruction of Alderaan and was referred to as a test, or demonstration.

Before that is there any information on the Death Star having been tested?

Answer

Yes, at least twice

The Death Star proper was tested twice before the destruction of Alderaan:

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  The first time, it was used to destroy the city of Jedha (on the moon of Jedha, orbiting NaJedha):

  
  

  

  
  

  
  

  The Death Star’s overbridge was dark except for the lit rows of instrumentation and the glow of the main display. Dominating that vast screen was what remained of the valley of the Holy City of Jedha: a churning, whirling, burning storm of sand and rock shards. The air, ionized by the energy of the Death Star’s weapon, flashed with lightning. At the storm’s epicenter, the crater of the incinerated city smoldered where the beam had sublimed the outermost layer of the moon’s crust.

  
  

  This was not the fate Krennic had envisioned for Jedha. The Death Star was designed to obliterate worlds, not maim them. Yet he wondered if the moon would ever recover from such an attack, or whether the cascading effects of a burning atmosphere and broken crust would result in a tortuous death played out across millennia. He felt in his bones that his weapon had exposed something profound—about the nature of worlds, about their lifeblood and their death throes—though he could not have put it into words. Maybe, he thought, that’s what poets are for.

  
  

  _{Rogue One: A Star Wars Story}

  
  
  

  
  


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  The second time, it was used to destroy the Imperial Citadel on the planet of Scarif, in order to prevent the Rebels from transmitting the Death Star plans (and, it should be noted, for Tarkin to get revenge on Orson Krennic, director of the Death Star project).

  
  

  

  
  

  
  

  The Death Star was pulsing with emerald light. Jyn tried not to tense. She wasn’t afraid of what would happen, but she didn’t want to suffer. Somehow she found herself closer to Cassian than before. Her breathing matched his, or his matched hers, deep and steady.

  
  

  The Death Star flared too bright to watch and a tremor went through the beach. The placid waves rolled higher, spraying flecks of warm seawater over Jyn’s cheeks like tears. An unfathomable rumble echoed ten or a thousand kilometers away.

  
  

  _{Rogue One: A Star Wars Story}

  
  

  
  
  

In addition, a superlaser prototype for the Death Star was tested long before the events of Rogue One, by firing it into a black hole.

Computer modeling showed the lasers’ twin collimating beams racing away from the Star Destroyer. Then, captured by gravity, the beams become one, changing vector and accelerating beyond lightspeed as it disappeared into the mask’s churning accretion envelope.

_{Catalyst: A Rogue One Novel}

It’s perhaps worth noting that in none of these cases was the Death Star tested at full power. The first two were in single-reactor mode, and thus only caused catastrophic damaged to the crust of a planet, rather than destroying it. The last, of course, was merely a few lasers fired from a Star Destroyer in the early phases of the Death Star project, not the Death proper.