Steps
1. Flirt with her. Start off light, so that she doesn't feel that you're being rude or going too far with someone who's already seeing someone else.
2. Make sure she is attracted to you. You will find this out if she tries to make body contact with you. Always look at a woman's body language; flipping her hair, leaning in towards you, making eye contact, staring at your lips, or touching your arm or leg are all good signs that she's into you. She will still have this body language if she's seeing someone else.
3. Make sure she has a good time with you. This is easier than it sounds; just keep things light and positive, while being respectful of her feelings and her significant other.
4. Express your interest. Maybe in a phone call or an ordinary conversation, let her know you're interested by asking her over to watch a movie she's into or asking her out for a drink. Let her know that you know she's dating someone else, and that it doesn't have to be serious between you right now.
5. Appreciate your friendship. Tell her that whatever she decides, your friendship doesn't have to change,
6. Be interested in how she feels about the situation. If she's comfortable with it, discuss how she feels about her current relationship. Discuss her views on relationships and dating, and ask her how she feels about dating more than one person at once. This should help you to find out where you stand, and it'll also help you to get to know her better.
7. Be prepared for rejection. Realistically, if she's already seeing someone else then there's a chance she'll turn you down. If things don't go so well, that's cool. There are other girls out there. She may seem like "the one", but there are many others that will make you feel the same way.
OR...
You could also ask him/her something like "so how's your boyfriend/girlfriend doing?" Usually they will answer normally and will let you ease in to coversation or let them alone, your preference.
