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Make sure you know each other a lot before you try this, or it will totally backfire!  Sneak up on her from behind, and gently grab her waist. Don't squeeze her skin or anything, just put your hands on her waist and leave them there. She may freak out a little at this––remember, someone has just sneaked up on her and put their hands around her waist! If she jumps slightly or screams and turns around, says, "It's only me!" and then smile. Do not laugh. Keep your hands on her waist. If she likes you, she'll smile back and maybe giggle a little bit, then return to her original position. Your hands should still be on her waist. Notice whether or not she leans in closer to you. If she does, tighten your grip ever so slightly, but then let go. If she is not interested, she'll probably just laugh and walk away. If you two are shy friends, she'll probably ask why you're doing this, and how do you explain that? Have a ready answer prepared beforehand, such as "I was just feeling affectionate". Note: only attempt this if the waist grab test was successful. Go up to her and hug her for no apparent reason. Make sure you hug her gently, not like you would with one of your guy friends. Put your hands on her upper back (steering well clear of her bra!) and slowly rub your hand in up-and-down or circular motions. If she's interested, she'll hug you back.  Notice whether her chin is resting on your shoulder. If it is, there's some chemistry there. Rest your chin on her shoulder too, and bow your head a little bit so it looks like you're looking at the ground. If she doesn't hug you back and moves away, she's not interested. If you're lucky enough to sit together in a class, attempt to hold her hand. But this has to be done very carefully. Make sure her hand is off the table and resting on her seat. If she's right-handed, sit on the left side of her. If she's left-handed, sit on the right side of her. Slowly move your hand towards her and gently touch her wrist. She may jump a little at first; remember, this girl may like you. If she does, try to stay holding hands for the remainder of the lesson. If she has a strand of hair in her face, gently tuck it behind her ear.  If she's interested, she'll likely smile. If she's not interested, she'll likely move away. If this girl does like you back, seeing you flirt with another girl will just make her feel stupid. She won't know you're doing it to see what her reaction is- she'll think you're doing it because you have a crush on the girl you're flirting with.
Try the waist grab Test. Try the hug test. Try to hold her hand. Play with her hair. Do not flirt with other girls to see what her reaction is.