There are very, very rare situations where a spirit is confirmed as being the 'ghost' of a dead creature. In fact, only two that I know of. Those two would be Zebulon and Dunkelzhan. In other words, two great dragons, at least one of whom died because he planned to become a spirit to guard the Bridge in the metaplanes.
For everyone else, we honestly have no clue. There are spirits who claim to be the ghosts of dead metahumans, but who can say if that spirit of 'Cleopatra' is actually Cleopatra's ghost, and not just patterned after a summoner's wishes at some point? This is the biggest reason why testimony of spirits is restricted in places that aren't the NAN.