Remember, 90-95% of communication between people happens through body language, if you want to spot attraction, look at her feet position, her posture, her pupils, her hand gestures, and if you’re already talking to her, then her tone, how much she’s laughing, her eye contact, her openness to physical touch, how agreeable she is, how much she’s talking vs you talking.
Many factors there I know, but with enough practice, you can spot these without even realizing that you paid that much attention .