Often, when checking out an abandoned site, you'll be faced with a perimeter fence. Maybe it's topped with barbed wire, or maybe you're like me and are just too lazy to climb. Remember that if a location is abandoned the local kids already know about it, and have likely already been inside. If there's tall grass, or even snow on the ground, wander around and look for paths or places where the grass is beaten down. Follow it, and will likely bring right to a ready-made and often hidden opening in the fence. This is something that has worked in several places for me.