The backer board is generally from the ceiling and down 18" to maybe 24". The paneling is very thin so when you tap on it it sounds hollow. When you tap on the area with the backer, the sound is much more solid.
What I did when trying to locate mine was to pull a switch and 12 volt outlet on the wall, then use a small mirror to look up inside. Then I took a measuring tape and ran it up inside the wall, using my fingers to hold the tape measure up against the inside of the paneling. When the end of the tape bumped the backer, I took a measurement at the switch hole and wrote it down. Then, on the outside of the wall, I place the tape measure on the same spot and marked the wall at the top end of the tape with a piece of masking tape. That gave me the bottom of the backer on the outside of the wall. I then measured down from the ceiling to the masking tape. With that measurement, I went to the point of where the TV mount would be, measured down the same distance and marked it with masking tape. Taping the wall verified that it was correct.
While I had the switches and 12v outlets hanging out of the wall, I mounted the TV mount, then wired the TV inside the wall, including the coax. Very clean install with no wires showing. Make sure of where the existing wires are on the inside of the wall, so you don't damage them while putting on the mount or doing the new wire runs.