In "Death in Heaven", Missy claims that she knows where Gallifrey is and gives the Doctor coordinates for Gallifrey. However, the Doctor later finds that Missy lied about the location of Gallifrey.
I began to wonder about the following question: if Missy would lie to the Doctor about Gallifrey, would she lie about her being the Master when she might actually be the Rani or some other renegade Time Lady?
The coordinates Missy gives for the location of Gallifrey are only half a lie. That location (10-0-11-0-0x02, as established by The Doctor in Pyramids of Mars) is where Gallifrey used to be, before the Time War and more to the point before the Doctor's trick to "hide" it. Missy's lie is that it's emerged from its pocket and returned to its original place, but the coordinates were plausible.
As for Missy not really being The Master, it can't really be ruled out, given that not only are Time Lords notoriously fickle with the truth, but so Is the showrunner! But there's little evidence for it. Everything about her plot in "Dark Water/Death in Heaven" fits with their longstanding relationship with the Doctor, and with the characterization of the Master as truly unhinged. The Rani, for example, was always a fairly calm and rational adversary, more amoral than immoral. No other Time Lord the Doctor's crossed paths with has ever been quite the same kind of murderously bananas, and none of them know the Doctor so intimately as to know exactly how to hit him for maximum impact!
Ten said something about Time Lords knowing each other even when regenerated. I think that Twelve recognized her as The Master but did not want to believe in it, so to be sure, he asked to her! She's the Master and the Doctor would notice if she was lying about it!
