I use Mrs is I know that person is married.
If I am speaking to that person and they look at an age where they may be married, I ask them whether they are a Miss or a Mrs, but subtly. As in "Yes Mrs... are you a Miss or a Mrs?"
Do not say "are you married?", english people may find that offensive or intrusive, especially if you do not know them well.
I use Miss if I know that person is not married, or they look quite young.
In writing I always use Ms. If I do not know whether the person is married or not.