`Make a comment about the holograms on the ID and tell her you want to see if hers has the same error as yours. but since you don't have your ID you cant show her the error unless it shows up on hers too.
or a friends once told you how to spot a fake ID, and even though you knows hers is real, you wanna see if the obvious things to look for are still on the ID (like usa usa usa being printed across the top). Obviously since you don't have your ID on you, you can't check yours, so you check hers instead.
or, you can ask to borrow her phone, turn it off, and then call it from your phone so that it goes straight to voice mail. Most people put their name on their voice mail, so you might be able to get it that way too.
Have you been to her house? Ask to see a old yearbook, or look for a piece of mail laying around.
Or you could just trick her into telling it to you. Tell her you have this feature on your phone(that you could never get to work right) where it will say the name of the person calling, and you want her to say her name into the phone so that you can record it and now you will know when its her calling. After you get her name, you can obviously get frustrated with your phone since you can't get it to work and then just say "aww **** it" and put the phone away. But now you have her name.