Okay so here's a tip that I didn't need to be told because I figured out of my own. Ask her what she likes. If you don't want to do that for whatever stupid reason, then casually point out a ring you saw and ask her what she thinks. There are subtle ways to handle it. "I saw a wedding ring that used a sapphire, what do you think of that" or "hey look at the ring in this picture I saw online," then listen to her opinion. If you do that a few times she might start to actually realize that you're going to ask her to marry you and give you useful feedback, assuming she doesn't state something useful in the meantime, such as "diamond would be better" or "I like that design but not the gemstones."

Of course if you haven't talked about that kind of thing already, clearly you're doing something wrong. My fiancee knew I was going to ask her to marry her, just not when or how. Well she figured out on the day that was coming because your girl is almost certainly better at picking up on subtlety than you are, especially when it comes to you.