I would also ask her. However there are a few things that you can watch for. Sometimes a girl that has another avatar will try to friend you with the other avatar to keep an eye on you. If you have another girl that suddenly comes on with your girl goes off, that could be her.
Watch your visitors closely, mannny times people forget to log out of their alt avi and accidentally visit your page. Look for an avatar that has the same badges as your girl, or seems to have the same likes/dislikes/things in common as your girl does. Most common girls do the same crap on every avatar they own, its easy to catch and see if its the same person, they also usually keep the same age as well. Visitors is your best bet.
However if your girl is smart, or a really good lair, its unlikely that this will help you.