i cant do any of those, its a friend from many years ago, we have just got back in touch and she remembers my birthday, but for the life of me i cant remember hers. and there is no one who knows her that i can ask.
You could get the conversation around to astrology and say "Remind me what you are again?" or you don't even have to be that blatant. She will mention what she is and you will have the month. You could say "I have a mate who has an ephemeris (me!) and she will look up your planets- honest, it's REALLY spookily accurate- give me your date of birth again" and write it down quickly before she has a chance to say "you know my date of birth"! Also, if she is on facebook her dob will be on that.x
Just say to her "You know, I feel really awful but I can't for the life of me remember when your birthday is!" best just to come clean, I wouldn't be too offended if it was me!
It all depends on the consequesnces of them divulging such (guarded?) info.
You can always casually mention that you are typical of your star sign.... or just ask them outright, explaining that you've never been good with numbers....lol