Smith is hard to do because of so many people with the same name. Since you know she was born in Virginia in 1749 you should look at the Church records, many of which are available for that time. If not the birth, then at least the christening should be recorded and show the parents. I would start with the county in which she was married, since it is possible she was born there. Check also the marriage record since it might say she was daughter of ______.
You might also look at abstracts of Viginia wills for people named Smith and look for mention of her or her husband, since she will likely be mentioned under her married name.
Good Luck and Happy Hunting!