Summarize:

You are about to take a big risk. However close you may be, and however compatible you are as platonic friends, going from friend to boyfriend or girlfriend is a huge change in any relationship – so huge that it will be changed forever. You should therefore carefully consider whether the risk is worth it. Can you get over a rejection? Are you willing to risk harming your friendship, even ending it, for the chance at love? You, and your friend, might not want that. Try to find out if the feeling is mutual. Does your friend seem to drop verbal or physical hints that she sees you as a potential partner? Does she flirt with you? Or, does she treat you as a sibling or talk about her attraction to others? If the latter, then she may be signalling that she isn't interested in you. You may be perfectly compatible as friends. You may get along famously, spending hours together one-on-one or in larger groups of friends, laughing together, and sharing all of your thoughts. But this does not guarantee you will be compatible as lovers. Do you share the same values? Beliefs? Passions? Will you have good chemistry? If you cannot determine this, can you honestly imagine that the two of you would work as a couple? Are you sure that your feelings are more than just lust or passing fancy? Sometimes major life events push people together and cloud our judgement. Are you on the rebound? Are you lonely? Are you both dealing with personal loss, like a death in the family? If the catalyst is one of these, you may want to wait and see how you feel in one or two months. Make certain that your interest is motivated by legitimate desire, longing, or love. It may be that she doesn't see you as anything more than a friend. You have to be prepared for that possibility. You also have to understand that your relationship won't be quite the same once you've come clean with your feelings. It may be awkward, or it may end entirely. If you have decided to risk your friendship, you have to be able to accept the consequences.
Consider the risks. Look for hints. Think compatibility. Be sure of your motives. Prepare for a rejection.