waterproofing shed wall from interior

Hiya, I'm fairly certain I have the center of a few studs, but since I need to put in some lag bolts and other hardware that is a bit thick, I want to be sure I have the center and not too close to an edge as to split or crack the wood when fastening. I'm guessing most would say to just use a finishing nail but I wish someone made a tool - a long pin with some sort of handle - that I could just push into the drywall easily and check to see if I'm hitting the stud or not. I can bang in nails and remove them - I know ;) But is there something simpler yet more convenient to use? Thanks

I own a home that was built in the mid 70's. It has in-ceiling radiant electric heat that is constructed by putting one layer of 5/8" sheetrock on the ceiling, stapling 240v wiring all over the sheetrock, and then covering that wiring with a plaster-like compound that creates a textured ceiling look. I know how this is put together because I had all the sheetrock on my ceiling in my downstairs family room fall down a few years back. Luckily nobody was down there when it happened. I'm assuming that the sagging section I saw finally came down and all the wiring helped pull all the other sheets down. It was a mess. I'm not sure why it fell. There was no evidence of water damage and they used nails and screws to attach the sheetrock. They were all left in the joists after the sheetrock fell. My problem now is that I have an area in the living room upstairs that is sagging pretty bad. It's an area about the size of two sheets of sheetrock. I know that I need to get into the attic to remove any insulation that has gotten between the sheetrock and the ceiling joists and then prop up the sheetrock. With the wires stapled to the sheetrock, though, I can't just start putting screws in. I need to be able to locate the wires and the ceiling joists from below. I have a nice stud finder that seems to be rendered useless by the plaster type material and the wiring. Does anyone have any ideas how I can locate the wires and joists to be able to fix this on a very limited budget?