Personally, I just address it on the envelope to "Jane Doe." If I know her on a first name basis, I start the letter "Dear Jane". Otherwise if I know her preferred title I use it (Dear Miss Doe, Dear Mrs. Doe, Dear Ms. Doe, Dear Dr. Doe...) If I don't know her preferred title, I use Ms.
ETA: I just re-read your post. If it's an elderly widow, I would use her husband's first name (Mrs. John Doe.) I can't stand that convention myself, but it's what was and is considered polite by that generation.