We have an underground and overground fence, but I don't know how or where I have a large backyard. How do I find where she's escaping from.

Have you walked the fence to see if there are any holes dug or parts of the fence torn open? What kind of fence material have you used, and what kind of dog is she? Could she be jumping the fence?

How high is the fence?
My little dog (13lbs) can climb a chainlink fence if you give him enough time...

First what kind of dog is it, and second how tall is the fence. Pit bulls which are only considered "medium" size dogs have been known to be able to jump 7 foot fences, so it is VERY possible your dog is jumping the fence.

Do you have a video camera?

Setting up a camera pointing at the most likely spot you think the dog may be escaping from would be a good idea. Even if you are home the dog may go to it's favourite 'escape' spot and give you a hint as to where and what he is doing.

We had to put an extension on top of our already 6 foot high fence to stop the neighbors shepherd coming into our yard.

Why not just watch him to see where he goes first. Usually when a dog knows where to get out, that's the first place they look and then go.