Which one to use?
When asking someone you dont know which is correct Su familiar es bien or Tu familiar es bien?
2 Answers
It depends on a lot. If you want to be safe, and generally more polite, just use usted. In my experience if someone wants to be refered to as tú, they will tell you, and be flattered anyways because you called them usted.
When asking someone you dont know which is correct Su familiar es bien or Tu familiar es bien?
You would ask:
¿Tu familia está bien? Your family is doing well? (to a friend you use the "tú" form of speech)
¿Su familia está bien? Your family is doing well? (to a stranger, boss, or elder you use the "usted" form of speech)