Nutshell version: Two-spirit is a term that comes from the collective grouping of folks known primarily as native american. In many, but not all, indiginous tribes (here and around the world) there was/is a healthy respect/fear/awe of those who share similar traits to folks that we would now call gay/lesbian/bi/trans/intersex/queer. The thinking was that if a man or woman displayed traits that would most typically be found in the someone of the opposite sex, then they likely had "two spirits" within them.

Here's a nice quote: "They were not like other men, but the Great Spirit made them winktes and we accepted them as such…. We think that if a woman has two little ones growing inside her, if she is going to have twins, sometimes instead of giving birth to two babies they have formed up in her womb into just one, into a half-man/half woman kind of being…. To us a man is what nature, or his dreams, make him. We accept him for what he wants to be. That's up to him" (Lame Deer as quoted in Williams, 1992, p. 25). Williams, W. L. (1992). The spirit and the flesh: Sexual diversity in American Indian culture. Boston: Beacon Press.

So how do you know if you are or not? It's really a matter of self-identification. Keeping with the "traditional" model, I would think that if you identified as part of LGBTIQ tribe, then you could wear the mantle of "two-spirit" if you desired. However you may be guilty of cultural appropriation if you do.