Take in consideration her daily activities. Is she active and always on the go? If so, the settings that are designed to protect a diamond are called Contour and Cathedral Settings.
Ask her closest family and friends if she has ever mentioned the style of engagement ring she would like. *This can be a great guide for you, plus one of her family members or friends could know her ring size.
What type of metal and color of jewelry does she usually wear? Knowing this piece of information can take the guess work out of choosing a metal for her ring.