I don't get all the fear and paranoia. Just ask. If you're honest and sincere, people know it. If you're creepy, they know that too.
Starbucks are always busy and tightly controlled. If you're going to ask a worker, just tell her she's got a great face and you want to see how she looks on camera. Suggest on a break maybe she could pose for a few shots, perhaps by the window or someplace with interesting light. Show her the pictures. No matter how good she looks, the camera may not like her. Some people are photogenic, some aren't. If she looks great, make some prints and give them to her. If not, just say thanks. If you both want to pursue it outside her work, that's for her to decide. I got on a bus one day and the driver asked me to take his picture. When he saw it, he wanted me to do formal portraits for his family.
The day I got my Sigma 35A, two years ago, I had some time to kill before shooting an evening event. I was in this bar/pizza joint when I saw the worker go past the window. When I saw the light on her I had to shoot her. It was mid-afternoon so it wasn't busy. So I asked her. She got her hair straightened up, checked her makeup a bit, and I posed her at the window. Never will I forget what she said when I showed her the picture. Surprise on her face, she said, "Wow, I really look good." That was the high point of the day for me. If I was paranoid about the whole thing, none of that would have happened.