All land in Alabama is considered Posted unless it is abandoned and not being used for some purpose. I don't know of any land in Alabama that isn't being used for some purpose be it timber, investment, or recreation. If you accidentally cross the line between your property and another's, you might not get a ticket, but if you have no land in the area, you will lose in court. In this case, the OP admits that he knows the land/pond does not belong to his friend so if he goes fishing and does not have permission to do so or the pond isn't open to the public, he is Trespassing. It doesn't get any plainer than this.