Just buy a few things from the store and talk to her casually. You know, "Hi, how are you?" -Insert her reply here- "Great. So, how far along are you?" -Insert her answer here- "Congratulations. Do you know the gender?" And go from there. If she seems friendly ask her if she would mind you feeling her belly after chatting for a few minutes. Do all this when there are no customers around of course, that way you don't hold anyone up.