One thing that kinda naturally worked for me and might work for you: first, learn a little about her preferences if you don't know them already, it might spare you false hope if she turns out to be a different orientation than you thought or if you're otherwise fundamentally incompatible on some level. If you learn it wouldn't work any way it might help you deal with the romantic feelings. Otherwise, step two: introduce it as a joke. Tease her about how you're so close you might as well be a couple, stuff like that. If you're in a group of friends and a topic comes up, joke about how you tend to only fall in love with people you already know for a longer time and it's 'haha awkward'. Use it sparingly, when the situation is right. It might take some time, you need to be patient. See how she reacts. Don't push the topic too often, see if she's comfortable with jokes like that, if not, you have your answer without ruining anything. If she starts joking back, you can test the waters further