Designing Kitchens and Bathrooms - Way to tell if backerboard is already installed

Hey, new to the forum. First time question here. I'm closing on an older home next week. The master bath was recently redone by the previous owners, they were an older couple. So they only put in a spa tub, no shower. They only tiled up the wall maybe chest height, since they didnt have a shower. Now, I want to add a shower head, and tile further up. Is there a way I can tell if they used backerboard up the wall and painted over, without just cutting it open with intentions of putting backerboard in. Hope that makes sense.

Kind of hard to paint over backerboard and not have it noticeable, without a heavy skimcoat. If you are adding to existing the existing tile, I would just go over the existing wall covering, that is not so risky since it is higher up.