I never saw a problem with going to jewelry stores to look at stuff. You can look at diamond earrings or pendants first and use that as an excuse to see what type of stones she likes. Easy to see if she has the same opinion about style of cut on an earring as on a ring.
There's nothing wrong with her knowing you intend to propose some day. They actually prefer that. You can still surprise the hell out of her.
Both ways can be good. If the engagement is a total surprise, she'll remember how you totally caught her off guard. However, if you do it in a way she doesn't like (like too public) or the ring bugs her, that can ruin any brownie points you got for the awesome surprise proposal. If you can nail the ring, every time she looks at her hand, you get those brownie points again. Figuring out what she wants and sacrificing a little surprise is the better choice IMO.