Mostly with yourself first, are you willing to risk a friendship for a possible relationship? Are you willing to possibly face rejection? Are you positive you feel this way?
Then, after you figure out all your own fellings, be honest with them. If you know you will regret not telling them your feelings, then there in lies your answer.
Hashing things out in a private and comfortable setting, is sometimes the best thing you can do. If you’re scared, remember, that this may be the love of your life. You have obviously built a strong friendship, and as long as you both keep things out in the open, then even if things don’t work out, you know you tried.
All you can really do is try, and then move from there.