Karen, you might start again by being a detective. You can't see into the walls from inside the room, but there are places where you can get a glimpse into your walls. These would be like in the attic or basement, sometimes in closets. If you can get an idea of stud spacing and estimate the starting point, you should be able to get a pretty good idea of where to drill a test hole or two to confirm stud locations. Then you can drill the rail for nails and run the nails into the stud. Your test holes should give you a good idea also of plaster thickness into the studs. One little tip is to slant the nails as you drill and drive them so you get 2 nails in an "X" shape into a stud. This holds the rail onto the wall better.