Well the easiest way to find out is to check Indian rolls. One example is the Dawes which enrolled Cherokee, Choctaw, Chickasaw, Seminole, and Creek signed between 1898 and 1914 in Oklahoma aka Indian territory.http://www.accessgenealogy.com/native/finalindex.php
That link gives you access to searching the Dawes, check to see if any of your grandmothers parents or grandparents appear on it. Also check Indian census records here on Ancestry and at http://www.accessgenealogy.com/native
there are a bunch of different rolls that you can check to see if you have an ancestor that was listed on it.