Here's how I see it. If you "pop the question," and she's not sure, the default answer is "no." I would first begin to feel her out with regards to marriage. Maybe something like, "Hey, did you hear that Jim and Evelyn are getting married? What do you think about that?" You know, fish for information on how she feels about marriage in general. If it's favorable, then at some other point in time, let her know that you've been thinking about it, and ask her if she ever thinks about it.
I know it's cliche as hell, but communication really is your best friend in any relationship. The important thing in my opinion is not to ask her in such a way as to present an ultimatum, but gradually ease in the idea and see if it takes hold.
Good luck, man!