You can usually get a rough idea from the context of what you are reading where the "Indians' are from. American Indians (Red Indians as they used to be called) are also known as Native Americans.

Also, nowadays "Native Americans" is a much more common term than "American Indians"; it's more politically correct I think. "Indians" makes people think of the older term "Red Indians" which is considered racist now. I'm not Native American or from the USA, that's just how I'd think about it.