Possession of the original installation disk is a good clue. And/or, so is the official sticker on the case if factory installed.
And even then, after the initial 90 days from first registration, the Windows Activation Technologies scheduler
(KB971033) may throw up its arms and call you, a character of fine upstanding, a thieving toe-rag of a pirate. Even possibly after the next 90 days, and again, and again.
Thus sending you into a tumult of emotions, searching for how you can assure Windows
(and yourself) that the genuine Windows product they
(or one of their fine agents) sold you is, in fact, genuine.
I'm guessing the bit about every 90 days ad infinitum, as, after it copped me out, and searching for help on the web, where a more amateurish
(must be, didn't know what to do) type of forum may shout "Pirate! Burn
(ban) him!"
(and they did) I found true help.
The more enlightened forums knew how to ask questions before "jumping" to conclusions. This is one of them. Windows 7 Forums another, Microsoft Answers yet another. seven forums
(spit!) wasn't.
So the good news at least is that if you get "stung" by this sudden accusation of piracy, and you
are innocent, at least you have come to the right place. You'll be aided in the cleansing of your fine character. And most likely be told to disable the W.A.T. scheduled tasks.
Peace and Harmony be with you.
(And if you are a pirate, may you burn in hell!!
)