Ask her friends if you're not sure. They will often know the styles she likes or exactly what she wants.
[header] How to buy an engagement ring [title] Find out your what kind of rings your intended fiancée likes. [step] Knowing the styles of engagement rings she prefers will help you narrow down the vast list of possibilities. [substeps] If you want the engagement to be a surprise, pay attention to her comments on rings she sees or when other friends get engaged.