“Miss” (Plural – Misses or Mlles. for Mademoiselles)
This title can have many uses, but traditionally, it is specifically for unmarried women. It is often used as a more familiar, casual title for close friends (e.g. “Mrs. Smith” might be called “Miss Linda” by her best friend’s children).
Whatever the case may be, you should call her by the name she prefers. Here is how I would go about it:
Just meeting, unsure of marital status: “Ms. Smith”
Aware now that she is, in fact, married: “Mrs. Smith”
On familiar terms: “Linda” (by you, if she prefers), “Mrs. Smith” (by the children), “Miss Linda” (by the children, if she requests it)
If you are addressing an envelope (or in a similar situation), always go for the more formal title. Even if your children (or you!) call her “Miss Linda” in everyday life, they should address her as “Mrs. John Smith” on their letters/thank you notes/etc.