I don’t know if it applies in this case (Iroquois), but often through the reconstruction method. Basically: if you find clearly related words across multiple languages you know to be related, you expect the ancestor of those words to be present in the ancestor of the languages.
Well, I guess we need an idea/rough concept, before we do the actual thing
Edit: on second thought, how do we even know that it predates writing, when there are no records of it?
Predates Iroquois writing, it should be said
I don’t know if it applies in this case (Iroquois), but often through the reconstruction method. Basically: if you find clearly related words across multiple languages you know to be related, you expect the ancestor of those words to be present in the ancestor of the languages.