Q: Once the house is wired, then wall papered, how to you know where the tape wire is?

A: In the un-decorated house you can see where the tapewire is (it becomes unnoticeable as soon as you have things in the room to cast random shadows…) and if you did all your layout with the layout stick then you’ll know where to look.

Shining a light flat along a wall shows the tapewire too and sometimes well enough to find it exactly with the testlight.  But if there is uncertainty or to make your placement perfect, then you can do a rubbing. Here’s a slideshow: (above)