How did Gandalf sneak into Dul Guldur?

How exactly did Gandalf sneak into Dol Guldur? You'd think that it would be patrolled by hundreds of orcs, both inside and outside the tower. Furthermore, how did Gandalf get all the way to the torture basement where Thrain was being kept? And how did he have enough time to talk for a few minutes with Thrain and recieve the map of Erebor? Did he hack his way through orcs?

Another question: How did Gandalf come to know that Sauron was 'the Necromancer' residing in Dol Guldur. Did he see Sauron? Did he overhear talk about Sauron?

I don't think there is any way to definitively answer these questions.