I honestly would just go straight to her and ask. She obviously has an idea it's happening so it's not spoiling the suprise. I'd simply say hey, I want to get the best ring possible for you so if you could just write down some preferences, I'd appreciate it. Price range, style, cut etc. She can give multiple types she'd like for alittle wiggle room for you and more suprise factor. But really in something this meaningful, it would be better to straight ask her whatever specific question you may have. Otherwise, your gonna hear yourself say outloud "I'm sorry honey, reddit said..." and well lol. Yea. Congratulations on your impending nuptials and I wish you both the very best!!