My sister is a lesbian, and I have known this for about a year now. She hasn't told me yet though. But when she does I'll just be like "sweet, so how are classes going?" It won't be a big deal.
Same with a close friend. If it is a male close friend, I'd trust he would know not to come onto me as I am hetero. If he didn't know better, I would tell him that I'm not interested.