Talk to your friends, because he probably will.
If your fella is planning on purchasing an engagement ring, he's probably going to covertly ask your closest friend, sister, or mom about your style preferences. Make sure they know which cuts, metals, and settings you prefer. Your gals might be very well-intentioned, but if they don't know that you are hoping for something princess cut and rose gold, you might end up with a very different ring than expected.
Draw attention to similar costume jewelry.
The first question the jeweler is going to ask him is, "What type of jewelry does she wear?" If you're looking for ways to leave hints, make sure you are your accessorized best on date nights and on your Instagram feed. Hoping for something platinum? Ditch any gold jewelry until after he pops the question. Also, it wouldn't hurt to "accidentally" leave your favorite platinum bracelet at his place.
Mention a friend's ring.
When a mutual friend gets engaged, mention her ring in the conversation (either with him or in a conversation he overhears) and whether or not you like it and why. A simple, "Her ring was really pretty, but I prefer gold," will get the point across.
Bookmark a page or two.
From leaving up a blog post that features your favorite ring styles to dog-earring an ad with a ring that you love in a magazine, it's totally possible to make your ring wish list obvious. Although, this is probably only something to do if you have discussed getting engaged. If you haven't, this is going to look more than a little presumptuous.
Don't stress about it.
More and more men are opting to postpone the engagement ring until they can shop with their fiancee. And unless he gets it engraved or orders something custom made, most stores have very good exchange policies. Also, it could be that the ring you have your eye on now will come nowhere as close to the one he presents you with.
Have a honest and straightforward conversation about it.
So, after all these attempts at jewelry subterfuge, we feel it's necessary to advise you that you can also simply have a conversation about it. If you've already discussed marriage and an engagement is imminent, there's no reason to not bring up the ring if you're already talking about it. Remember that this is a gift, so he might want to surprise you with something he picks out. However, if he would like your input, be straightforward and reasonable within his budget.