How to tell if a girl's enjoying the back massage your giving her?

What things will a girl do to show you that shes very relaxed and enjoying the massage your giving her. I gave my good friend a neck, shoulder, upper and lower back massage. I gave it to her because i wanted to relax her. I knew her lower back was really soar because she was on her period. I never communicated nor did I ask if she wanted the back massage, i just went ahead and did it. She didnt move away, instead she settled herself in, which is a good thing... But what are other signs that girls show that they're enjoying the massage?