Question: know if girl has boyfriend?
Answer: She's Not Dressed Up All the Time. ... 
She Has Pics of the Same Guy All Over Social Media. ... 
Her Body Language Suggests That She's Taken (e.g., Touching the Guy She's With Or Facing Inward When With Her Friends) ... 
Pay Attention to Where You Are and What She's Doing (Such as Buying Men's Clothing at the Mall)