That depends on whether "someone else" loves her in return. If so, you hardly stand a chance, but you probably should tell her to realize it yourself. It'll be a compliment for her if you phrase it like one: "I know you're happily in love with X, but I just have to tell you that I feel attracted to you."
If her love is unrequited, just tell her to try you instead, because you DO love her.
If she's just mooning (unconfessed love) you could try prodding her into it to see how much is behind it. Or you can behave as if you didn't know and ask her for a date.
I am sorry to say that if she is in love with someone else, then please let it be. I know it must be painful to you, but telling her how you feel would only hurt her as well and serves no purpose. If she loved you, she would leave whomever she is with on her own accord. I don't really see that happening.
I think most of us have loved someone and they haven't returned the sentiments and it hurts, but there are so many wonderful women out there I am sure you will find true love for yourself.
**I know that it hurts a lot to see the person whom you love ,love somebody else. The thing you can do is stay patient and be confident that she will some day understand your love and will surely respond .Do not ignore her , this will make her just feel bad. I have gone through this kind of situation.I cried for days but my friends help me up.
I know it is hard to say her but if you really like her and want her then you should try once.Go to her and say to her that you like her if she want you then she will accept your proposal if she not then forget her.