There are lots of different ways ways to pick the perfect engagement ring. She’ll be wearing the engagement ring for the rest of her life, so you’ll want to make sure you get it right. One option is to propose without an engagement ring, and then go to pick it out together. While this is a sure way to know that she’ll like the ring, some people feels this takes some of the romance out of the engagement.
You might consider bringing her best friend or mother shopping with you, or have a romantic date of preview shopping for the ring. Another option is to look at what kind of jewelry she wears - from what kind of metal, to whether it’s contemporary or classic, even perhaps “borrowing” a ring to take with you to the jewelry store, which will also help you know her ring size.