Agree with Velvetee. If the woman hasn't remarried, she probably still retains the "Mrs", as it'll be on documents such as her passport, etc. If you feel uncomfortable about using it, then just "J. Smith" or whatever.
Either you could use Ms. or just her first name and last name. Mrs. is also appropriate,but I think it is a bit awkward as you are the present Mrs. "smith" or whatever, so how could you call her by that title without feeling awkward about it. If it were someone else it would be appropriate to call her Mrs.so-and-so.
Despite what people seem to believe these days, the title of Ms is not officially for divorcees. It's meant as a way of addressing someone if you don't know their marital status, nor want to use their christian name. in this instance, I think Ms is the most appropriate way to address her and you will be using it for its correct purpose.