how to find information

i’ve recently been tracing my ancestry, and have come upon a great-great-grandfather who the family says was blackfoot, he was a coal miner in pennsylvania, my grandmother says the eastern part, near new york. i’ve requested that she dig up marraige licenses and such (which she says she has) to get any names, but how do i go about finding out if he was a member of a particular tribe?

thanks,

jesse finn