If the girl you are with is interested in you, she will exhibit certain signs of attraction and arousal. Each little mannerism, for example the sparkle in her eyes, her tone of voice, her body language should be taken into consideration when trying to decide if the lady you are with is genuinely interested in you.
Signs Of Arousal
Wide, easy smiles accompanied with a relaxed face. She blushes frequently and her skin tone become redder.
She maintains eye contact throughout and her pupils are dilated (they become larger). She would often blink more frequently than usual and her eye lashes flutter. You might even notice a sparkle in her eye. The eyes are windows to the soul and are one of the most accurate ways of telling if she is attracted to you.
Subconscious biting of the lips or showing of the tongue, wetting her lips or even chewing her fingernails.
While talking, she is slowly stroking her cocktail glass (or any cylindrical object) up and down with her thumb and index finger. (reveals what she is thinking of, if you get my meaning) She might also be fiddling with her jewelry.
She starts sitting up and her arms aren't limp. If her crossed leg is pointing in your general direction and rocking back and forth.
She adjusts the tone and pitch of her voice to match yours. In fact, any sigh that she is mimicking you can be taken as a sign of interest. For example, laughing in unison or crossing her legs if you've crossed yours.
She winks at you in the middle of a conversation or even from a distance. This means she really is interested in you. Even exposing the palms of her hand facing you show that she might be interested in you as it has been proven to be a subconscious sign telling you that she has nothing to hide.
She twirls her hair around her fingers or perhaps strokes her hair lightly with her hand while maintaining eye contact.