Johannus said:

Yeah maybe, just something that is visually incomplete, but yet still clearly daunting. We would see from the destruction of Alderaan that it is operational, but like between scenes you would imagine the rebels taking the attitude that its best to stop it before it is finished.

yea, but by Episode IV, all they were doing it getting weapon systems online and testing them.

Read Death Star Novel: Talks about how there first test was on a planet they had been orbiting. blew away half the planet.