The safest way is to get someone qualified in to do it.
Assuming the wall is a hollow plasterboard stud wall.
If I were doing it and I wasn't sure what was behind the plasterboard then I'd gently cut a square hole out about the size of a pair of electrical sockets. You should check that there aren't any electrical sockets of any kind on a horizontal or vertical plane in line with where you want to cut, on either side of that wall before making a hole of any kind, if there is I would recommend you seek professional services to carry it out. That way you'll be able to see what's behind that section. The key is not to put the plasterboard saw too far into the wall but just enough to go through the thickness of plasterboard. If there's nothing in the way you can then drill through the other side. Then refit the square of plasterboard you've cut out by fitting a small piece of flattish wood across the hole from the inside by tying some sting to the piece of wood to hold it firm whilst drilling a pilot hole through the plasterboard into the wood either side of the opening. Secure with screws through the plasterboard into that piece of wood, then remove the string and then drill some pilot holes through the square piece of plasterboard into the piece of wood bridging the hole. Once done and the screws are tightened so the heads are just below the level of the rest of the wall, a little red oxide primer on the screw heads and then once dry some filler over the screw heads and the square cut line. Rub the filler down with a sanding block once dry and then roller over some matching emulsion and you ought not be able to see where you've been except for the drilled hole passing through the wall.
I'm routing the long way because most of it will be hidden behind various bits of furniture. If i go the other way it'll have to go around the door frames which will be much more noticeable.