In the fifth harry potter book how does harry find out if sirius is safe or with voldemort?

Sirius was never with Voldemort. Sirius had never left Grimmauld Place, but because throughout the novel Harry and Voldemort had a stronger connection with Harry experiencing/"seeing" some of the things Voldemort was doing without him being aware of it, this lead to Voldemort setting up Harry...Harry needed Oclumency to block not only himself from getting into Voldemort's mind, but vica versa. Harry, being as stubborn, naturally as he is thought that not only did he not need Oclumency (not to mention, quit taking lessons with Snape after the Pensive/Worst Memory incident.), but by sneaking into Voldemort's mind occasionally without him being the wiser(which he was continuously warned by Hermione wasn't such a good idea and Dumbledore obviously had his reasons for why he *needed* Harry to block his mind). Harry thought he could use this information to his advantage, which wasn't entirely true and ended up backfiring against him. Because Harry got into Voldemort's mind and witnessed his attack on Arthur Weasley Arthur's night on patrol in the Department of Mysteries, Harry was such a strong presence in Voldemort's mind, Voldemort not only realized what Harry had been doing but twisted it to his own advantage. He found out that Sirius (as Dumbledore said), was the *only* person that Harry would have gone to the Department of Mysteries to save. Irrationally, wholeheartedly, without a plan, and without thinking. He loved Sirius very deeply (again as Dumbledore said) Sirius loved Harry "Most in the world." Harry had no idea Voldemort had tricked him until he was trapped in the Hall of Prophecies with the Death Eaters (and his friends), then he knew he was foolish and lead astray. This was even more so confirmed when Lucius Malfoy laughed and goaded Harry, by saying "He [the Dark Lord] knew you would come...He always knows" or something to that affect (paraphrasing from memories years past when I read the book last; I don't have the Order of the Phoenix in front of me!). There's the full, complete answer to your question...Ultimately by falling for Voldemort's trick Harry led Sirius to his death...He was also tricked by Kreacher, and so many had a hand in the cirstances, but Sirius was "safe" and once Harry was at the Ministry and Sirius was informed of Harry again being in unnecessary danger, he set out to save his Godson; mistakes were made (again, by many) and his life was taken suddenly/tragically by his cousin, Bellatrix Lestrange.