1. I don't tend to walk up to people and call them by a racial classification, or address people based on a racial classification. ESPECIALLY people I don't know. I don't go walking down the street and see someone and go "Hey White Guy! What's up" or "How you doing black man"? Similarly, I'm not going to go up and say "Hello Redskin".
2. I recognize that it can be viewed as offensive, and I also recognize as it's an anachronistic word at this point, so if for some crazy reason I was randomly deciding to identify someone I've never met by a racial category I would likely use the most benign and commonly accepted method since I'm DIRECTLY referring to them which heightens the chance to offend. This is much the same way I'd refer to a black individual I was addressing in a similar fashion by using "African American" even though I routinely use "black" instead of that, because I recognize that's generally viewed as the most benign and neutral term.
3. Its a largely dated word in the current vernacular, and as such doesn't tend to come to mind in terms of racial classifications nearly as quickly or as naturally as something like native american or indian or american indian. It's similar to "colored" for a black person; it's dated, it's considered offensive by many to use it to describe a black person in the modern day, but it's still recognized in some cases as being used in a non-offensive manner today due to the context of those cases.
Now if it's NOT a complete stranger, but rather a friend, it'd greatly depend on their personal feelings regarding it. If I know them to have no issue with the word and I needed to reference native americans I wouldn't have a huge issue using it, though likely wouldn't because once again it's rather dated as a method of referring to native americans in every day vernacular. Not out of any concious thought, but simply because saying "native american" or "indian" or "american indian" is simply what comes to mind and the tongue quicker. I associate words like "native americans" and "indians" with a race, I associate the word "redskin" with a football team, and thus when talking about the race my brian is FAR more likely to spit out one of the other terms 99.9% of the time. If they did personally have an issue with it's use I likely wouldn't use it around them, out of respect for a friend.