You would want to take an autosomal test to test for admixture. They can detect Indian ancestry within 5 generations. However, because the amount of DNA inherited from grandparents back is random, it is a good idea to test other cousins, if none shows up. It is also possible that the ancestor already had a lot of admixture, even though he was a member of a tribe.
The three firms doing these tests are:
http://www.23andMe.comhttp://www.23andMe.com $289 (includes a health component and gives Y- and mtDNA haplogroups).