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.
We did something like that. We both had talked about getting married, so we were both on the same page (this is very important). We both went and looked at rings, decided which we'd want and which sizes fit us, then I bought them and asked the jewler to engrave the date of the proposal. The wife wasn't allowed to see, obviously, so she only knew that it was going to happen *someday * but not when.
The date went great and she said yes. Everyone happy.
I recommend my colleague to do the same. He did not. Now he's trying to return the ring which is both too big and wrong style.
Over heard it says that the engagement itself should never be a surprise, just the when and where.
Oh and I'll add in to listen to her if she says she wants/doesn't want something specific. If she doesn't want it to be a big public thing, then don't propose on the stadium fan-cam. If she likes hiking, do it while hiking (but not somewhere the ring can fall in a river or off a cliff). Stuff like that.
I found out i was getting married when my wife asked, "is it weird i am looking at engagement rings?" I was like, "haha. I guess we are getting married".