Unfortunately I believe each unit is wired by hand, and two identical units might not have their wiring done the same way.
So, I can't think of anything that will be 100% foolproof. You might be able to use one of those wire detectors, but I believe the can only detect live wires, you could still possible miss finding any wires that aren't producing an EMF.
In my TT, I just try to follow the same common sense I do in my house. Near plugs, lights, etc. you can expect to find wires. Aside from that, look to see where those wires would logically go. For instance, in a pop up, if all the wires are in one corner to run down a single support pole, then the far opposite corner most like won't contain any wires from that light in the middle of the roof.
There's no use crying over spilled milk, unless it's on your keyboard.