i would NEVER buy perfume as a present (unless the person had specified which one trhey wanted) even when they have one, they might just be using it up and not really like it
If you want to go ahead I'd suggest one of the younger designer perfumes (not celebrity) such as the new Boss Jour (the one advertised by Gwynneth Paltrow). Even if she doesn't like it she should recognise its quality from the brand.
There's only one perfume which suits all skin types, and never alters to smell of cats ( which, alas, some perfumes do on some skins)
It's Mitsouko. An old, old classic. And it endures, it doesn't fade nearly as fast as modern ones.
Coccinelle's answer is very good, when buying jewellery for a 21 year old I sought out a very young sales assistant and took her advise. Recipient was delighted with 'my' choice.