In my limited experience (which practically makes me pro given the sad state of most of us) I found that a direct question like that only works if your damn sure she'll say yes, and if it becomes very apparent that you are going to do it sooner or later. I'd recommend saying it as a long day or evening or whatever of activity together is winding down. Maybe not at the very end if you can help it, you'll want to enjoy your accomplishment for a little while. If she really likes you than such a simple thing won't matter, even if your a dumbass like me and say "what?" when she nodded and said "mmm hmmm", whats important is what you did to build it up.

Good luck OP!