Question about tv backer

[Rogerl1016]

We recently bought a used Wildcat 28RKBS. The factory installed tv had been removed by the previous owner. I'm wanting to install a new tv but don't know how thick the backer board is. The bolts that came with the mount are 3 1/2 inches long. This is for the tv that goes into a wall with a linen closet on the other side of it. Can anyone help? I don't want to run the lag bolts through the other side. Thanks.

[camper007]

Maybe slide a small nail or piece of wire through previos hole's and see how far in they go.