The non-verbal cues are when she begins to squirm around, trying to position her breast beneath your hand or her head begins to toss from side to side as you work your magic. These cues will allow you to file her likes and dislikes away for future sessions. At this point, you can do one of two things. You can either continue with what you are doing or you can stop and move on to something a little different. I don't mean stop touching her breasts, simply change the stroke that you are using or focus on a different part of the breast for a while.
Anticipation is a wonderful thing.
The actual touching of the breast can be done in a variety of ways. Once again you will have to experiment with your partner to see what works best for both of you. You can begin with just lightly skimming your fingertips over the fleshy parts of her breast, completing ignoring the nipple. Move your fingers across her breasts from side to side or move them in a circular motion starting at the outside of the breast and working your way in closer and closer to the nipple. Let your fingertips just skim over top of the nipple, then resume your previous strokes.